Quality boat building ply is very different from WBP or so called marine ply. It’s to do with number of ply’s, timber quality, surface veneer thicknesss, voids etc. you probably don’t need it. Robbins Quality is excellent the prices extortionate. There is a company in Falmouth supplies similar quality at a much cheaper price but still expensive.
I would suggest that for your job you can find decent veneered ply at a decent price. Don’t bother about waterproofness they are all water proof but look at the veneer quality. A lot of marine ply sold in Builders Merchants is rubbish and I wouldn’t personally put mdf anywhere on any kind of boat it doesn’t stand damp very well. A lot of ply from diy stores is also rubbish. Try to find a timber merchant dealing in different plys. Don’t buy without seeing it!