There were lots of discussions last year about float devices. One guy made a foam yoke that would keep the phantom on the surface, but it almost always (from memory) turned upside down. Another guy attached 1 litre water bottles vertically inclined under each motor and this sort of worked, but the aerodynamics were awful and the aircraft handled like a pig apparently.
The general consensus of opinion was that it wasn't worth it for all the extra issues that the floats would cause.
As mentioned above, the phantom doesn't know it is over water - only the operator does. It can be very disconcerting to fly over water - tell me all about it; I lost a P1 into the sea (video link in signature below) but the best videos in my opinion are filmed when the camera is able to film from a perspective that Joe Public can't reach easily, and the best of these are over open water.