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I'd use whatever small camera bag you'd normally use, and take along a small waterproof pack liner or dry bag to cover the camera bag when required. They both work the same way, with a roll top, but the pack liner is considerably thinner and lighter, as it doesn't have to cope with abrasion, as it's normally used on the inside of a bag. You should be able to compare alternatives at the REI and MEC web sites. OTOH, if you want to have it spray proof while you're taking photos, then there are different alternatives.