If you want to take your camera in the water, but only inadvertenty under the water more than a few inches or so, the waterproof bag with hard plastic over the lens will do fine. If you're doing anything with the camera below a meter or so, consider a hard case: it's the water pressure that will kill a camera in a baggie: 5 feet down, you're applying nearly 2psi to that nice LCD on the back. I use the baggie type to keep the camera clean on the beach.
Voice of experience here: if your chosen solution doesn't float, consider tying yourself to the camera and/or attaching a very bright ball to it: it's invisible once you let go of it, and it sinks faster than you can.