John, With all respect to your safety concerns, don't put in expandable foam with the intention that will assist your self/rescues. A kayak with proper floatation (bulkheads or floatbags) will still be unmanageable should you not be able to get the water out of the cockpit. If foam is to be at least as useful as bulkheaded compartments then you will have to have the entire compartment full of foam otherwise that cubic foot or so of foam will only be useful should the entire front or back compartment become flooded. In that case that cubic foot of foam can displace water, but should that occur you will be in a mess o' trouble from a rescue standpoint. The only reason I can see to have a cubic foot or so of permanent flotation in the kayak is in the event of abandoning a totally disabled kayak that may get recovered ,,someday. Your attention would best be applied to making sure the hatches cannot become dislodged is any scenario and rely on float bags that fill the entire compartment for back-up. As a seperate concern I think foam could trap water after a few years and discolor a varnished kayak, but I'm guessing on that.