Red Fisher,
The myth of an "airtight chamber" in a wooden boat is just that, a myth. A builder can seal it up all he wants, it will expand and contract with changes in temperature and air pressure. As this happens, it may actually hold air - for a while.
Soon, all the flexing - it will happen several times daily no matter what you do - will weaken a joint. Now, air that is sucked in and pushed back out will leave behind condensation. Mold, wood rot, etc. will follow. It may take a few years, but it will happen.
Construct a bulkhead, add a water proof hatch, AND drill in a very small hole up at the top of the bulkhead. About the size of the period at the end of this sentence. Now, as pressure changes the air can flow and equalize within the chamber. If you capsize, water may flow all of two or three drops through the itty-bitty hole.
You now have a well designed, usable piece of equipment. In use, store gear or nothing in the chamber. And, when you store the boat, pull the hatch and let the chamber vent to open air.