Check out [link]http://www.foamfloats.com/[/link] for floats. Floats should be about 75% of the fuse length. Engine should be ok. What prop are you using. You might have to go to a 11x7 or so.

Couple things to remember...take off into the chop...let the plane come on step before giving it elevator...don't force it out of the water, you will need the ground speed to "pop" the plane out of the water...the water will try to hold the plane down...most important....HAVE FUN AND ENJOY WHAT YOU SEE...oh, one more thing...figure out a way to retrive the plane if the engine dies in the middle of the lake.