My first boat (14' jon), the deck was 1/2" plyood untreated and it got warped. I replaced that with 3/4" and treated it with some stuff like Thompson's Water Seal. That worked well but the 3/4" plywood weighed the boat down more. You might not really need 3/4" but it has better rigidity than 1/2".

On my boat now (15' modified V jon), its a more heavy duty boat so I stayed with 3/4" and treated it again. I gave it three coats on top and bottom with whatever water sealant was cheapest at the hardware store. It was some kind of spray on stuff that dryed fast in the sun. Its been two years now and one of those years the boat was stored outside, and no problems with warping or anything.