PRESSURE. Without some way to mitigate the wild pressure differences, the race can only exist on a single 'layer' of ocean. Also, haven't you thought why intelligent life developed on earth on, well, Earth?
2. No, they can't, because they have no materials for them. They can't have chains because they can't heat metal.
3. Yes, exactly. Without metal, every single task will be greatly hampered (from construction to farming) and any progress involving spacefaring is rendered simply impossible.
4. They need materials and technology for bodysuits. Neither of which they have because they only have access to whatever grows on the bottom/floats on the surface/is reachable from the surface.
PPE:
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FUU
For phosporus, we need to have both phosporus on the planet and the means of extracting it in an era when we don't have
anything. Literally. Unless there's something sufficiently hard for tools on the ocean bottom, we're completely stuck.
Also, I just thought of another thing. For civilization, you need farming or some other way of getting a
crapton of food while at the same time freeing up people to become builders/artisans/scientists/warriors/whatever. This completely destroys the possibility of life anywhere above the bottom (unless you build abovewater farms) and if life exists
there, it'll take an eternity for them to reach the ground (because pressure), another eternity to start smithing (because they're completely unfamiliar with metals) and... well, comparatively not too long to actually get out into space. And by then they aren't underwater.
The only way I see any form of water-based civilization working is if we have a surface-living race that makes its food on islands or at least shores and has a
good reason not to get the hell out of water for full-time living on dry land. Oh, and it must have a non-existant on Earth material that is sufficiently hard to make tools (and, later on, spaceships) from.
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FFFFFFFUUUUUUUUU
Why did hippies win if Obin have as many votes?