A sea sock is a piece of safety equipment used in sea kayaking. It is a large waterproof bag that is placed inside the kayak and attached tightly all around the rim of the cockpit. The paddler sits inside the sea sock and fits the sprayskirt over the sea sock and cockpit coaming as usual.
A sea sock is different from a spraydeck, which keeps water out of the kayak while the kayaker is in the boat. If the kayak capsizes, the kayaker can exit the capsized kayak underwater. The spraydeck remains attached to the kayaker, but a sea sock remains with the kayak and prevents it from filling with water.
A sea sock also keeps the kayak free from sand and dirt, as well as reducing instability caused by water sloshing around inside the kayak.