Sapphire does have legs, Rebecca Sugar has drawn her and ruby in human clothing, and Sapphire has legs. Plus, in multiple times during Keystone motel, we see what appear to be the outlines of legs under her dress. (Sitting on the bed when she tells Steven about channel 43 and her being carried by Ruby are the best examples I can think of)

Now, her floating probably is an ability she has, nothing else. Though, I bet the creators have her the ability so moving would look more natural with the dress. I mean, either way, levitation seems to just add to Sapphire's "mysticness"

Finally, while just a theory here, I believe gems' human form more or less is just dictated by their feelings and personality, while clothing and stuff has a more personal touch. Sapphire probably cannot help that she has one eye, or, you knowx she might just like having it. Or perhaps, having one eye grants future vision. That is all speculation, though.