It just so happens that I'm doing this very same thing (except it's on poles, not in a tree). The floor is just cheap 15mm ply, not exterior grade. I've ended up buying a tin of Marine Gloss to seal it with because it's weather proof and can be used in high-wear areas.
An alternative is to use Bondcrete. Hovo reckons the ply floor in his boat has lasted for 3 years so far after sealing with 50/50 bondcrete, so you could give that a go.