Hi Rapid ratter,
The 15l version of these . . .
http://www.facewest.co.uk/Exped-Waterproof-Packs.html are pretty good, 100% waterproof and fairly comfortable too. My partner swears by hers and uses it on a regular basis, rather than a fuller, better padded pack. Mine often rattles around in the bottom of a sand-filled sea kayak and both have borne up well to the abuse we give them.
Andy
PS - Exped also do dry bags that you can pop into a conventional rucsac and check out Ortlieb - they do dry bags for cameras and more general use too. All good and fairly strong stuff that lives up to a life at sea or in a canoe on the river

!