@TheNerdInTheCorner I think you could build underwater out of stone, what would knock it over, water currents? I think interestingly storms wouldn’t be a problem if you were quite far down, they would mostly blow over the top. Digging caves is another possibility.
I think there are some underwater structures from areas which have flooded.
Re reasons you can’t make an underwater CIV I think the main things are (I’m not an expert and open to other ideas or corrections):
You need metalworking to make an advanced civilisation.
You can’t make a regular furnace underwater.
You can’t really use hydrothermal verts as for one thing there’s not really enough of them to support a large civ. Another thing is they’re not hot enough, you need 700C+ to melt metals, to melt iron you need 1700C+.
In contrast to the approximately 2 °C (36 °F) ambient water temperature at these depths, water emerges from these vents at temperatures ranging from 60 °C (140 °F) up to as high as 464 °C (867 °F).
I don’t think anyones seriously suggested you could try to find lava etc. Anything above 407C would be really hard to handle as the water would start to vapourise (at least that’s what wikipedia says) which would make the are very dangerous.
Apparently you could use electroplating to make metal coated tools but I don’t think that’s going to get you far enough to build a rocket.