>I've been toying with the idea of using empty PET soda bottles for floatation
 
When I bought Umiaq she had plenty of floatation in the form of empty gin, rum, whisky, vodka, etc... bottles that were filling (completely) the under-cockpit section. It had taken true dedication to provide that much flotation.

Giles Morris
Arlington VA
Vancouver 25 #002 "Dolphin"
Montgomery 15 #236 "Umiaq"
Sundry kayaks