it ain't just in one direction,
> but in all directions, and will create a suprising amount of presure
> inside the ends of the boat. I seem to remember someone being worried that
> the pressure in the ends of the kayak may cause the two halves of the boat
> to separate at the ends, especially if they didn't get the tape all the
> way into the ends.

Yes,by all means be careful with the expanding foam.A few years back a friend decided to "soundproof" his van by pumping expanding foam into the body panels.This turned out to be a HORRIBLE mistake.As it turned out, he used too much foam and ended up with a very lumpy, disfigured, virtually worthless vehicle.