I haven't had any issues with warping of plywood from the big bog store. My issue with their plywood is de-lamination. Even the domestic stuff is low quality and overpriced.
I have started using baltic birch for everything. My nearest supplier in San Antonio, TX has 3/4" 5'x5' sheets for under $30 a sheet so its very economical as well.
Sikkens. Doesn't really form a film, but lasts for years. We've tried everything on the market on docks here. Nothing else (yet) comes close.
It's much thinner, and runnier, than water. If you put it on with a roller, without watching what you're doing, you'll lose half of it down the cracks. Using a thin roller, dribble (more like a run) a bead down the middle of a board, and spread it around with the roller, not trying to soak the roller very much at all.
If the deck is not very large, use a brush on a day when you have a light breeze. Without the light breeze, you will get a light head.
I haven't found a pump sprayer yet that works to suit me with it.