When I wrote about my hiking and backpacking essentials last summer, one of the first things I put on my list was a dry bag — a waterproof, airtight sack that keeps your belongings from getting wet.

Nothing spoils a weekend camping like a wet sleeping bag. Or a wet tent. Or soggy oatmeal, a waterlogged lantern or squishy socks. Thankfully, avoiding any of these misfortunes is a simple matter of ...