Elodea is great, but it isn't floating, Egeria densa and Egeria najas do better floating than actual elodea. it probably can grow floated, but it grows roots quickly. Never had E. densa, but I got egeria najas and it is a weed. I saved it from destruction from fishes by putting it in a pickle jar full of daphnia. Grew from a single tiny piece, on a windowsil, some of it floated. WOW! Now I have tons and I donated a ziplock bag full to the LFS, still have the corner of my planted tank full of it.
Hornwort doesn't care what temperature it is. Elodea does a bit better in cold water, but egeria najas is a warm water plant.
I like utricularia vulgaris, if you can get it. Easy to grow, floating, carnivorous. WHOA! Looks like milfoil sort of. In the wild, it gets 4 inches across, but in my tanks it is less than 1.5 inches in diameter. It is probably because there is less plankton... but anyways... +1 on hornwort. It doesn't float that well for me, it is sort of neutral density. Maybe not making enough oxygen to float.