The name kind of says it all. DRY. BAG. You really shouldn't need much more than that, but just in case (see below).
Soft and flexible rip-stop fabric with watertight roll-top closure for maximum compression
Polyurethane coated with watertight, double-stitched, tape-sealed seams for professional waterproof
It can carry food and drink, clothes, wallet, cell phone and other personal belongings