Well, the three (this is take three arter all) prevailing solutions to the underwater civilization problem are:
1 amphibians, they could forge above water or enslave a terrestrial species but mostly live under water.
2 rafts, a aquatic species may stumble across fire and make rafts to store it.
3 bubbles, they can be sealed in a storage container and filled with the air from photosynthesis or swim bladders, both of which are oxygen.