Plenty of backpacks can handle some water. Filson's Submersible Backpack can go underneath it. Crafted from fully-waterproofed 840-denier nylon, it has a large, waterproof main Tizip opening, with a clear zippered front pocket for gadgets, a water-resistant zippered side pocket, and mesh pockets on the sides, shoulder straps, and interior. The hip belt, shoulder straps, and back panel are padded with breathable, mesh-covered foam, and D-rings can be found at the back, sides, and each shoulder strap, with Molle attachment points on the front of the latter allowing for even greater customization.
Height: 18.75" / Width: 10.25" / Depth: 8.25" / Capacity: 28L