From Kayak Wiki
An in-hull wall that seals off a compartment in a decked boat, used for flotation or for dry storage accessible through a hatch. Also what you can buy at certain big-box discount stores in Nevada.
The bulkhead is a slab of material (usually similar to construction i.e. glass, plastic, wood) that is fitted to stop water entering in to a dry compartment within the kayak. The usual configuration is one to the front of the kayak and two to the rear; one creates the rear hatch and the other a day hatch. This has been advanced recently and some kayaks now come with two day hatches and even hatches to gain access to dry areas above your knees. [by Richard Cree, Ocean Paddler Magazine - Issue 008]