Absolutely. Considering that life as we know it is dependend on liquid water, it´s surprising that aquatic aliens are fairly rare in sci-fi settings in total, in virtually nonexistent in Stellaris. But with only 1 water world type, it would not be well balanced. So yes, new planet types first, aquatic aliens after that.
Aquatic civs will have a difficult time utilizing energy, if they are any similar to earth's fish. They would not know fire, smelting ore, refine metal to make new tools. They shall rely on physical force, or the flow of the water to achieve these, which is nothing compares to fire. If their planet has land, nature will push some aquatic species to go there and become un-aquatic, either from avoiding predators or seeking new habitats. just like on earth.
Then again stellaris as a game should not dwell on it too much, they look cool, and it's will be a cosmetic only DLC(hopefully), if one find them immersion breaking he/she can always not buy it.