We start with gill fish-like animals, which has a prehensile mouth with deft fingers/tentacles.
Their invention would start out with the creation of stone hand-axes. These axes would be found in all nations of this animal, including those who live in swamps. Being swamps, there are many trees around, and therefore a lot of wood in the form of fallen or broken branches. In some part of the world they find that this wood, much like Earth’s Bald Cypress, is resistant to the weakening effects of the water. This wondrous material, with all of its uses, will quickly spread around the swamps. Soon, someone will want to go straight to the source. They figure that if an axe can cut at a single point, then a line of axes will cut a line. Hence, by glueing a line of small stone blades into a straight stick with some sort of resin or sticky animal secretions, they invent the first saw. With this, they saw down a tree of this alien cypress. Now, they are the masters of wood.
Contemporaneously with this, peoples from the marshes discover races of quick-growing annual hydrophytes which produce large seeds like eggs or grains. These seeds last a long time and are edible, so quickly they become a staple food for these people. Through casting some of the seeds back out into the fields, their food supply is continuously renewed. Then somebody notices a trend: Plants with small or few seeds produce more plants with the same traits. They then propose to eat only these poor yields, and throw back only the best seeds. This leads to the plants quickly evolving, their seed-stalks growing from mere hairbrushes to bending bundles of bounty. These plants are traded away, allowing agriculture to spread over the sea.
Another sort of agriculture will take place, that being animal agriculture. Various peoples around the sea will end up finding useful animals, and some might be tamed. With this tame population, they’d be able to breed these animals and get more of them. Eventually the same realization will happen amongst them that happened to the plant-farmers, and soon domestication will follow. Through trade, these domestic animals will be brought throughout the sea.
When wood enters the global market, things really start to heat up. With new tools being created with wood, agriculture grows more and more efficient, and settlements start to grow more and more. In these new bustling cities, there are countless well-fed people, with a nice long winter where there’s plenty of free time to go around. In this environment, people start to specialise. Various new jobs are invented, such as spinners, weavers, waxers, and seamsters. Transportation will be ever-more important, and so wagons and other such vehicles may be devised. Writing also will be invented for recording things like trades and farming.
Another thing that might emerge in these great cities is philosophy. Mostly it’ll be of a practical variety, trying to work out how their world works. Some may focus on the body, finding an outline of breathing and the benefits of aeration. Others could study air and water, perhaps even learning about the heating affects of friction in air. Yet more may ponder on the boundaries of the world, such as the land beyond the shoreline. Eventually, in some nation, one of these curious philosophers will get the ear of a powerful leader, and be able to direct efforts to their goal of seeing the surface.
A waterproof material will be called for, which will motivate the invention of a tough fabric holding a hard wax. The fluid philosophers may help here by devising a sort of covered heat drill to melt the wax into the fabric, forming a more resilient composite. These philosophers will also have the task of allowing for aeration. This may lead to the invention of bellows, made of the waterproofed fabrics. Explorers and traders find animals with tough yet transparent shells, which are caught in great numbers and delivered back to be formed into windows. Animal trainers are employed to train their amphibious draught animals to go onto land and respond to visual signals, so that they can be controlled on land.
With this knowledge, the supermarine is built: This is essentially a giant open waterskin, with wooden bones to support it. Bellows were built into it to aerate the water, and windows were added to the sides through tough, heavily-waxed wooden pegs. This contraption was placed into a great wagon, drawn by a team of strong amphibious draught animals. A team is assembled, and the supermarine goes to land. If all goes well, they explore the surface world, making all sorts of observations. But information is not enough for the people, so the rulers promise that the next trip will come back with physical riches.
The field of mechanical engineering has been developing throughout this time, and so the rulers decide to draft some philosophers of that specialty into their plans. These philosophers manage to contrive a small rope-operated crane with a gripper-hand to go in the supermarine, to which a trailer is also added. This second voyage marks the first interaction these aquatics have with the terrestrial world. When they return, trailer loaded with exotic plants and odd stones, their work is much more appreciated. And with the aid of the mechanic philosophers, their tools and attachments can become much more elaborate. For example, an upper-class philosopher interested in how these airy things react to heat might be able to get a large heat-drill powered by an animal engine. With such a tool, they could investigate heating a variety of objects.
Eventually, they will try and heat up a light, dry bundle of plant matter at the end of a long, sunny excursion. Then, they will discover fire. It won’t last long with just the tinder, but it will definitely leave an impression on the philosophers. Replicating this phenomenon (and making sure to survive it) will be a major field in philosophy. It won’t be hard to find out basic facts, such as that it doesn’t form with moist materials or that it can consume wood. They’ll experiment to see what else a fire can consume. Most likely, they start with cheap, mostly useless materials such as coal and weak wood. Through this they will eventually learn that coal can burn. Another thing they might experiment with is clay: While it won’t burn, they’ll notice it go hard and strong when placed in fire. The manufacturing benefits will be immediately obvious.
Now, the supermarines will go from curiosities and philosophical instruments to practical machines. Their tools will be improved and specialised for the creation of clay objects such as bricks or pottery. However, philosophers will still have some access to the surface. The study of fire will continue, using these new clay implements to their advantage. Pipes might be used to investigate the effects of ‘aerating’ a fire, and brick structures could be assembled to find ways of defending against the heat. Eventually by combining these ideas in the right way they’ll invent a furnace. They’d at some point notice that these structures produce a distinct flame, so they may experiment more with these fires. When they decide to throw an ore of some metal into the pit, they’ll discover metal.
Having invented an entirely new material, they’ll need to figure out what it is. At first they’d only have metals like tin, zinc, or copper, but they’d quickly have the idea to mix them together, thus inventing alloys like bronze. Through this they’ll start their belated bronze age. Now metal has become all the rage in the manufacturing world. Metallurgy will become more elaborate, transitioning to iron bloomeries, and on to blast furnaces, until they learn how to make steels.
With so much industry on land, there will be efforts to colonise it. With wood, metal, matured clay bricks, and perhaps some waterproofing agents like wax, they will be able to create a large tank on land, in which people can live. This will make their terrestrial industries much more efficient.
However, throwing a bunch of sea-water into a tank isn’t exactly good for the long term health of the occupants. They will soon realize that they need to set up some life-support systems in their cities. One thing they’ll want to do is find ways to support nitrifying effects. Once someone realizes that these reactions are caused by organisms, it’ll be easy to work out a method to cultivate these colonies, such as aerated frameworks of spongy matter through which the water of the city is pumped. With regular filters added to these, their water should stay clear of ammonia buildups. Nitrate poisoning is still an issue, but they should soon realize that plants will help them avoid this issue. With this, they should be mostly self-sufficient, only needing food imported, or a top-up of water.
There will also be other investigations into fire, namely relating to chemistry. While some chemical reactions will have been recorded in water, the airy environment is a much better place for chemistry to progress. Through this, many new materials, such as different fuels, will be discovered.
While this is happening, a philosopher of mathematics has an idea: Calculation, though quite important, is a boring job. Seeing mechanical philosophers designing ingenious machines to ease manual jobs, they theorize if the same could be done for computations. Working with a philosopher of mechanisms, they devise an ingenious, though impractical solution. When metals become commonplace, this revitalizes their work, and they find a way to make their machine. Though it is slow and clunky, it can perform calculations on its own. With refinements and more minds working on it, a turing-complete mechanical computer is eventually devised.
Another discovery is electricity: Static charges would be discovered over the course of experimenting with materials above the waves. However, this won’t be very exciting until metals come along. A pot containing multiple different metals, if it touched a metal component on a supermarine, would produce a spark. Such an event would be investigated, soon leading to the discovery of the electric current, and the galvanic cell. With this, the study of electricity will commence. Unlike other fields, once they have an insulator (such as wax), they can experiment with electricity under the water, allowing for many advancements in the field. With so many hands in the field, soon someone will think to see how things like lodestones or magnets interact with these currents, and through this experimentation they will at some point discover dynamos, motors, and other electromagnetic tools.
With all of this discovery above the surface, philosophers will want a way to deal with these phenomena up close and personal. With a few mechanisms they can assemble a small, personally-powered supermarine, with the wheels and bellows powered by a tail-crank. The important part is the mask/glove at the front, which allows the user to directly grab things on the surface. This is the first time in history that an object on the surface has been held in someone’s hand. They do various experiments with the cooler phenomena of land, such as electricity (doing things in open air is much more convenient). Experimenting with different metals in electricity, they will stumble upon the design of a simple point-diode. From this, it won’t be hard for them to discover the transistor.
Now that solid-state electronics are known, people will start to look for applications. Some mathematician might notice this, and think that these new components might be useful in a computer. Working with electric philosophers, they create the first electronic computer. It’s not very good, but it’s much quicker than its mechanical predecessor. With this amazing new technology, so many more things are possible. Obviously, the design is iterated upon to become quicker, more powerful, and more versatile.
At some point after their experimentation with fire, they will notice that fire can lift up things like light cloth. Through investigating this phenomenon, they will come upon the concept of a hot-air balloon. As their knowledge of the world expands, these balloons will become better, perhaps eventually enough to lift a personal supermarine or a set of electronic sensors. Philosophers will notice that a bigger fire will be able to lift more, and when metals come on the scene they’ll incorporate those into their designs. Through this line of reasoning, someone will manage to create a small rocket. With this new method of ascent, innovation will follow: Better fuels, more suited for rockets will be made. Chemists may also discover a way to extract aluminum or titanium from their ores, which will be a wonderful material to make their rockets from. As they make bigger and better rockets, the utility of such endeavours will be recognised, and electronic sensors will be sent up to examine the sky.
Soon they will realize that their atmosphere is thinning as they ascend, and so they will have to ensure that their rockets can continue to burn without air. The chemists should be able to work something out, allowing these rockets to fly higher, perhaps even reaching space.
The point will come when these people endeavour to send a person into space. With this, philosophers turn to inventing a way to remove CO2 from water, without relying on gravity. A solution could be to continuously cycle the water through a spinning heated cylinder, with a vent into the middle to extract the gases. With this system, plus an oxygen tank and parachute, they will be able to send people to space. Thus marks the first time these beings enter space.
Once they have successfully sent someone up into space, they will continue to develop their travel. Just like with their conquest of land, they may start to construct a space station to sit permanently in space. Unlike rockets, which can afford to store up their oxygen, these stations need a constantly renewed supply. They may be able to achieve this by purifying their habitat water and electrolysing it for oxygen, as is done on the ISS. Like the ISS, the hydrogen can be reacted with the CO2 to regain some water back and to eliminate the CO2. The rest of their life support should be doable with the same methods as in their land cities. When they reach another planet, such a system could be replicated on the surface to create a surface colony.
With this, they have well and truly reached the Space Stage.