Most boating or camping/kayaking shops should be able to sell you a waterproof pouch that you could put in any bag you like.

You can get waterproof bags to pretty much any size you like, we used to sell 60lt bags to put inside 60lt rucksacks for canyoning etc.

The option of putting a waterproof bag inside what ever you intend to use may make it all more useable and cost effective, one water proof bag can go inside all sorts of other types of bags, be it backpack or whatever you want.