Falcon Trading sell a few plastic planes. these planes come as a floatversion too, floats included!
There's a trainer (Named Ready), a Cessna and a Beaver, all high wing designs. Bulletproof and 100% water resistant, I'd go for one of those.
(I have the beaver myself and it's a pretty good flyer)
http://www.falcon-trading.com/
Make the necessary mods to protect electronics from spray and go fly.
Make sure you have a small boat to retrive the plane if you crash or the motor flames out.