Sealing the plywood before hand will cause at least a couple of problems that I can see.  First, it will make the wood much heavier and harder to work with when the time comes.  Secondly, and more importantly, it will make for a lot of extra work, because anywhere you will need to use epoxy to assemble different pieces together it will require you to sand those areas in order to get a good bond.  

I have built at least two kayak kits made from Okume plywood where the wood had been sitting stacked for years in less than ideal conditions.  One kit was kept in the rafters of a carport, in Olympia, WA for 3 years where there are extremely moist conditions.  I noticed no warping of any kind or any other problems.  I don't think you will have too much problems as long as you keep the wood from getting soaked and if you can keep it either flat, or well supported.  

If you can keep it away from liquid water (which will leave permanent noticeable stains), dirt, gnawing rodents, klutzy humans - and store it flat, or vertically well supported - your ply should be OK.   On the other hand, if you have accessible suppliers, you might just buy what you need as your project progesses, and have no worries at all about damage.