The ply I get locally is birch 3 ply. I bought 50 square feet today. I've compared it to the lite ply I used on the last runs of kits, and frankly it is more than up to the job.

I can cut 1/4" interior ply in one pass at partial power. Cuts pretty straight with the 50.8mm lens. I bought a 100mm lens too - I cut some 1" foam for giggles, works great.

Water resistance is likely not a big problem.. by the time the ply is in trouble from the water the balsa would have probably left the scene anyway?