Ever need to carry extra stuff on your board or in your kayak and want to keep it dry? Dry bags (or sacks) generally have roll-over tops with a fastex clip buckle. Most are made from vinyl or similar materials to be waterproof and abrasion resistent. A few have windows so you can see your stuff inside, essential in finding that one item you need. For attaching to a board, look for bags with daisy chain straps on the side to run cam straps, a carabiner, or a rope through to better secure down. Kayak deck bags can sometimes be waterproof and are great for SUPs as well.
Best brands include Ortleib, SealLine, Seattle Sports, and Outdoor Research.
Here's a short video I made showing the Outdoor Research Double Dry Sack, which has two enclosures on one bag to ensure dryness. I've recently used this bag extensively to carry my camera gear in my kayak cockpit while revising a Pacific Northwest kayaking guide for Mountaineers Books.