You can find some pretty good, inexpensive, waterproof boat bags by googling around. Look for tool bags. As for zippers, I wish there were saltwater capable, corrosion-proof ones on all the gear that’s sold for flats fishing. You can buy very expensive rain slickers and shoulder bags, made ostensibly for anglers, that are throw-aways after six months of flats fishing.
That’s why serious foul-weather gear for commercial fisherman doesn’t have zippers.