If I have a float plane, can I land on any body of water?
It really depends on the state, though you can't land in National Parks (the National Park Service regulates that, not the FAA).  The FAA doesn't care where you land it, though if you ball it up due to poor choice of landing area then they'll have something to say.

Some states don't really care where you land (like Oregon), others don't let you land anywhere (like New Jersey).  You should contact your state and get the regulations from them.