For anyone who enjoys activities where you're guaranteed to get soaked, the 28-litre Aquapac Toccoa is a good way of keeping essential kit dry.
It has an IPX waterproof rating of 6 (this is an international measure of the 'degree of protection provided by enclosures') which will protect the contents against heavy seas and temporary flooding - if conditions get worse than that you'll probably have a lot more than wet gear to worry about.
You simply stuff all your kit inside the single compartment, fold the top over several times then fasten it down with the side straps and away you go; or with full loads there's the option of top closure which gives you more internal volume.
There's additional external storage in the form of two mesh side pockets and daisy chain webbing to which objects can be secured, and a fairly basic shoulder strap and harness although it's good to see that there's padding on the back of the pack for your upper and lower back.
Clearly the Toccoa is a relatively specialist bit of kit - you wouldn't really choose use it for regular hiking and such (although you could) - but for kayakers, cavers and anyone else who regularly finds themselves lugging their gear around in very wet (or muddy, or sandy) environments it's a great way of ensuring you can do so in comfort at the same time as keeping everything dry and clean.