A:For something larger, check out SealLine's 55 liter Black Canyon Dry Bag ($55). It’s the size of a standard hiking backpack, which means it'll hold most of the gear you might take on a typical trip. It's made from a tough and light urethane-coated nylon, and, like Sea to Summit's bag, it uses a a roll-up top and buckle system to form a seal.
For some items—an expensive camera, for example—I'd urge you to consider a hard case like the Pelican 1450 ($150). The Pelican is made from super-tough plastic, seals out water with neoprene O-rings, and is filled with egg-crate foam to protect from drops or knocks. And as an added bonus, you also can order it without foam or with padded dividers.