It's always good to seal the wood around the engine to keep it from getting oil-soaked. Everybody has a recipe, but a mixture of epoxy and denatured alcohol is hard to beat. Also, polyurethane varnish is immune to almost everything after it dries.

IF you anticipate flipping over the trainer, it will float on the wing with the nose in the water. Coat the inside of the fuselage with sealer as well, at least up to the trailing edge of the wing. Trainers are great on floats, but sometimes the high wing catches a crosswind and pivots the plane around so the wind will flop it over.

A plastic bag/baloon/condom around the receiver/battery helps keep the water out, but any water that gets in stays there. A squirt of "Corrosion-X" is a good idea on your Rx, Battery, switch, any circuit boards, and connectors. Available at marine stores or at www.corrosionx.com/

Mount the receiver in the middle of the fuselage, so that it won't get wet if the plane is dunked, and it won't get wet if there is some water sloshing around inside.

Mounting your switch inside the plane so that it is actuated by a push-wire is also prudent. In=off, out=on. If you do in=on, you will arrive at the flying site with dead batteries when the plane gets bumped in transit.

Some advocate attaching the engine to a substantial airframe component via a lanyard so that if a serious crash happens, you have a greater chance of retrieving the engine instead of having it sink. I don't do that, and the only time I lost an engine was flying a pizza box over the lake....I screwed up, it hit the water, and coroplast does not float so I lost the entire plane.