Personally, I never waterproof the inside of my seaplanes - and the reason for this is simple.

You're never going to get ALL of it waterproofed, so if gets wet, the water will seep under the places that ARE waterproofed.

Since those places are sealed, the water will take forever to dry out.

Just leave it bare wood. If it gets wet, take the wing off and leave it in the sun for a while and it will dry out. The basic construction should keep it from warping, and besides, with the exception of a major mishap, you shouldn't get TOO much water in there (And in the event of a major mishap, water is the least of your problems)