But why would they prefer such a world to an ocean planet? If WWIII happened and Earth became a Tomb World, humans won't suddenly find irradiated wastelands more habitable than a fresh new Continental planet. Over thousands of years, sure, a fish species could evolve to live on land, but then it wouldn't be a fish anymore.
At least with Molluscoids you can say, "it's an alien mollusc who evolved in a different climate". But an aquatic species is, by definition, aquatic. Putting aside biochemistry and different atmospheres, which are ignored in Stellaris, it makes no sense that they'd turn down an ocean planet in favor of, say, a dry climate.
Mind you, I'm not opposed to aquatic species. I just hope that it's thought out and rationalized in-universe.