Make sure that it is labelled as water resistant. These varnishes are based on PVA resins I believe, some are not waterproof. If you are not over-concerned about finish, quite a good approach is to use a waterproof PVA glue, slightly thinned down to a brushing consistency, and apply several coats. In the dim and distant I made a couple of "skim boards" for the kids out of 6mm ply and applied several coats of this to seal them against sea water – it worked fine for several holidays except where the sand scratched through the coating..