I can't sell the game at all, anywhere, while it has art from FTL. If I do, they will just sue me.
It's hard to explain all the details behind the descision to make the game underwater, all I can say - just wait till it's out, and then judge. Also I might release a space version of the game after the first one. (I also like the space theme very much).
Also, while I'm looking for the artist, I will keep making updates with new feature, and will be releasing them for Tachyon.