Ok, let's regroup in this for a second. First off, the premise that started this conversation was the assertion that without bulkheads, a strip kayak that got swamped would sink. My response on the need for flotation plus bulkheads is based on the premise that that's true - but let's get out the envelope and see.

Nick very helpfully provides some numbers - he estimates the weight of a 1/4" WRC strip plus 6 oz layup at 0.7 lbs/ft2. At 48 strips per foot, that's a density of 33.6 lbs/ft3. As the density of fresh water is about 62 lbs/ft3, that gives us a positive buoyancy of about 30 lbs/ft3 - so the premise is false, the bulkhead-less swamped boat is still going to float. Using his numbers for Guillemot, I estimate the total amount of material at roughly 1 ft3, which means the boat has about 30 lbs of positive flotation. A heavy hand with the epoxy might bring that down to 20 lbs.

For a 3/16" strip, it's a little denser. I subtracted the density of WRC (23 lbs/ft3) to get the density of the layup (10.6 lbs/ft3) and then backed into it with 64 strips/ft rather than 48, which gave me a density of 37 lbs/ft3, (.57 lbs/ft2) yielding positive buoyancy of 25 lbs/ft3, or about +18 lbs for a Guillemot built with 3/16" strips. Note that a heavy hand with the epoxy is going to be much more significant with the 3/16 strips than the 1/4 ones in terms of density.

That said, a 1" thick sheet of 2lb foam delivers 5 lbs of buoyancy per ft2 and weighs .17 lb. Not sure of the surface area under the deck, but if it's 5 sf, then a layer of foam cut to shape and attached with velcro to the underside of the deck so you could remove it if necessary would add 25 lbs of buoyancy (a bit more than a type I PFD) and weigh less than 1 lb. Seems like cheap insurance to me.