There are a number of aquatic species in the galaxy that have achieved spaceflight (such as the fashan and kokatik) but the aquarians are actually a transgenic species of humanity which were part of the slow-FTL starship DSE Poseidon. The Poseidon arrived at the star Lacaille about 12 light years distant from earth, finding that Lacaille VI, dubbed Leviathan, was suitable for habitation as expected. The people of Poseidon came ready; transgenic modifications were already part of the plan to make humanity functionally capable of living on the water world.
The aquarians survived for two and a half centuries before contact with the Terran Order was established, and they rebelled against Terran control for ten years before the Terran Order fell apart through civil war. Today, the aquarians are a valued addition to the commonwealth of man.
The deaders keep their homeworld secret, and probably for good reason; a lot of species are frankly terrified of the parasites, and although its been determined that they require dead tissue to reanimate and puppeteer (and not living tissue) the mere thought of walking, talking preserved corpses puppeteered by a small centipede-like monstrosity is enough to send most species running.
The deaders keep to themselves for the most part, avoiding contact except in the fringe worlds, where they are often a bit bolder in their interactions with other species. Some deaders are known to have formed mercenary corporations, and offer out their services as unusually effective PMCs to the highest bidder. In places like the Hexen Expanse they find themselves welcomed with open arms, regarded as especially desirable for their skill at arms and the deleterious effect on morale their mere appearance induces.
Stats: Deaders and Aquarians use the default base stats as described in the Science Fiction Companion.