I used to paddle quite a bit, both whitewater and touring, and I have accumulated a whole mess o' dry bags over the years. Many shapes and sizes. They are VERY useful, as has been stated already by many. I think I have mostly Sealine (there's a hyphen in there somewhere) but I'm pretty sure I have some other brands as well.
Generally, they are made of PVC/vinyl and are often re-infroced with woven fibers to
reduce punctures and tearing. A good way to protect them further from rough or pointy things is to stow and carry them in a cordura-type duffel bag.
For some reason, there are always a lot of rocks wherever I happen to be whitewater paddling.

Rough rocks are generally not friendly to any water-tight material, so if I ditch and lose some gear, I want it protected from both the water AND the rocks. Sleeping in a soggy bag is the pits, man.
I always kept everything in my Alice in appropriately-sized zip-locs, tall kitchen waste, or lawn-sized Hefty bags. Always carried a few extra of each, too. (A split-open trash bag made a great water-proof barrier for the overhead cover of your fighting position, and a field jacket in a garbage bag made a heckuva pillow.) But, with all the rough-and-tumble that a ruck gets, the bags would often get punctured or torn, and even the zip-locs would pop open. Heavy rains (infantry weather!) would find any chink in your plastic-wrap armor and exploit it. Soggy socks are almost as much of a drag as a soggy sleeping bag.
So, I recommend getting yourself a few dry bags, a few different sizes, and then get something tough and abrasion-resistant to put them in. It will be a little heavier, but you can't find better protection for your gear in anything but a Pelican case.