Side panels are made from cheap fabric.,
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This review is from: Overboard Prosport Dry Backpack, 20 Litres (Sports)
It is a bit misleading to call it a dry backpack, which would give the impression that (like a drybag) you could immerse it in water for a long time with no worries.
The problem is that while the front and back are made from some thick rubbery stuff that is never going to leak, the side panels are made from cheap nylon fabric with a plastic coating on the rear. These will start off waterproof, but with use and time, the backing material will detach and the fabric will leak.
For protection against rain, I think it will be fine. It is probably not the right thing if you want to go swimming with it.