If you are going to be near water, either backpacking, camping, swimming, going to the beach, boating, water skiing, wakeboarding, fishing, or kayaking, you've got to protect all your stuff from getting wet. You know - the stuff that absolutely can't get wet like your camera, cell phone, wallet, and dry undies.
Check out this very practical dry bag (it means it's waterproof) made by Sealine. They make many different kinds of bags, but these Dry Seal Roll-Top Closure bags are great because they're made with transparent vinyl. So if you are looking for something in your bag, you can see right through the plastic to make sure it's still in there.
The bottom is made out of heavy-duty vinyl to protect against abrasion. All you do is place your things in the bag, roll down the top a few times (to keep the water out), and then you easily secure it closed with the D-ring closure.
These see-thru dry bags come in 4 sizes: 18 oz ($23.95), 14 oz ($19.95), 10 oz (16.95), and 6.5 oz ($13.95). They also make other kinds of bags that are much larger, so check out the Sealine website if bigger is what you need. If you are planning to be on or near water, dry bags are a total necessity.