Anything can float, even concrete. The college of engineering at my Alma Mater used to build concrete canoes every year and race them.

It's Archimedes' principle that's behind floating vs. not. In basic terms, if the weight of the water displaced by the volume of an object is more than the weight of the object, it will float; otherwise it will sink.

You have two issues to deal with if you want to try to sink-proof your plane:
1) waterproof the electronics by either sealing the compartments they are in or sealing the components themselves
2) Make sure that the total sealed volume of the plane displaces enough water for the thing to float. If the fuse and wing fill up with water then they are no longer displacing water. You could put sealed air filled bags inside the wings and fuse so they wouldn't fill with water for example.

Interstingly, you're not the first to have this idea. One of the unsuccessful pioneers of powered flight was Samuel Langley. He had the idea to launch his "aerodrome" from a house boat on the Potomac river. Just to be safe, he also added air-filled tanks to provide flotation in case the thing went in the water (which it did).

[link=http://www.flyingmachines.org/lang.html]http://www.flyingmachines.org/lang.html[/link]

Mark