Their species name is Z'narnian. It says so on the 'The Aliens' link page. Derek and Val's species name is Dorak (and the little shadow monster guy who likes to try to crush people with rocks, his name [not species name, just regular name] is Chaz!).
Ah yes, they definitively cannot fly! (They can't float up /that/ high off the ground)
I kind've imagine it working like they always have this invisible little bit of water floating around 'em that they can swim (or 'float') in whenever they want hehe. ((That isn't actually a thing btw - just how I like to imagine why and how their floating biz works))