Float bag

From KayakWiki

Float bags provide critical flotation for all kayaks. Kayaks without hatches and bulkheads must have float bags and/or a seasock. A kayak that can completely swamp runs a high risk of the Cleopatra's needle scenario which is difficult, if not impossible to rescue. If you are miles from shore, this could mean life or death.

Float bags are a good idea as a back up to hatches and bulkheads in rough water or any situation where the hatches could come loose or dislodged and the risk with a flooded boat is dangerous.

Float bags are necessary gear in kayak surfing and whitewater not only because they provide flotation but also because they displace heavy water. For example, a 60 gallon kayak swamped with water can weigh nearly five hundred pounds. In the case of a wet exit, the addition of float bags not only increases the chance of recovering the kayak but also reduces the weight and consequently the damage caused by collisions in the surf zone or white water. Even with float bags, a swamped kayak is a dangerous surf hazard, or a long swim for a whitewater kayaker! Surf, rough water, and whitewater kayakers should develop a bombproof Eskimo roll not only to increase their enjoyment of the sport but also to protect themselves and other wave users from a dangerous, out-of-control swamped kayak.

See Make your own float bag.

See also Surf hazard, Swimming.