Although I am aware that there is a mollusc class of species in the game, as well as a mammal that is described to have inhabited the seas for some of it's history, both of these only appear to be former aquatic races that eventually immigrated onto land. So far, there don't seem to be any proper sea-inhabiting races.
Maybe there are purely aquatic species, but we don't know.
There are some problems and questions on how a fully aquatic race could develop all the technology needed for space-travel, but Stellaris doesn't claim to be 100% realistic, and a marine spacefaring race doesn't seem much less likely than FTL travel being developed by several unconnected species within a small timeframe in the same galaxy(at least to me).
Well, as prudent people say, we are the only case of space-faring species we know, and we didn't even go very far. It's entirely possible that non-aquatic space travelling species are the exception. People who say that it's harder for an aquatic species to reach space are unimaginative or anthropo-centrists - water environements doesn't even need to be the same as what we have on earth. We couldn't even imagine some kinds of aquatic monkeys in aquatic forests and steppes. I won't write a book here, but the sacred "stone tools -> fire -> wheel" (or its variants) is not necessary. And you don't even need to imagine complexe organic technologies - fire can be replaced by various chemical reactions, for example. So, as usual, it's better to trust the rule of cool than the rule of pseudo-realism.
These aquatic species would, however, obviously require some mechanics to be changed/adjusted for them(e.g. adding a different class of tiles for submarine environments, changing some planetary invasion costs, etc.), so I see why they wouldn't be in the base game(due to time constraints). However, I feel that an eventual DLC for this would be high on the interest-lists of many people. Post your thoughts on the topic here, as well as any suggestions in regards to the mechanics themselves for these races.
Each planet has its own very specific environements. Great storms. Big deserts. Giant trees. Solid clouds. I think it would be better to link it to the planet than to the species - For ocean planets, the species would live either under the sea, or on floating islands, something like that.
There is no reason an aquatic species which formed an advanced civilization wouldn't be able to reach the stars. However it would have been a very different experience then our own.
That's entirely true.
Maybe relying on organic technologies more then inorganic.
There are many plausible mechanisms by which an aquatic intelligence can accumulate energy from natural sources, practice the chemistry needed to transform, concentrate, and control it. Therefore the answer is yes: an aquatic civilization could conduct space travel.
I think there road to space travel would be a longer one but once there i think they would make the adjustment to space travel easier then we would have. It certainly would make things easier for them if they lived on a planet with lower gravity then our own considering space vessels filled with liquid would be substantially heavier then our own. Of course initially they might go into space in space suites with there environment within having to replenish what they breath periodically.
I don't understand why our own road to space travel should be shorter. Especially since technological is only a prerequisite - we didn't go into space because we could, but because of political reasons. And we didn't even invent FTL space travel yet. Maybe aquatic species have something that ease the process, and maybe they are discussing on their own forums : "Can terrestrial species into space ?" and ther answers are "it's obviously harder for them to develop FTL because they don't have schlourves. Their road to space would be longer, as obviously they can't benefit from the water protection in space and have all kinds of problems with low gravity." etc.
I think it's as difficult for us to imagine alien species that it would be to understand astrophysics without being able to see anything else than their own planet. They wouldn't even be sure that there are other planets. And remember how your prediction were wrong about exoplanet... This time, we already thought that our solar system was a good example, and it looks like it's in fact an exception. There are gaz giant next to the sun, and other weird things. I don't think we are able to understand things as complexe as intelligent life or the road to FTL travel without more examples. When we talk about aliens, we need to keep in mind that's almost pure speculation (well, except for the obvious things, but we need to be very prudent.)
