Scrub near the split with coarse sandpaper to give the foam better adhesion. Use a stick to reach deep into the kayak if necessary.

Use ropes and straps to bend the kayak back into shape and force the edges of the split together. Maybe use a correctly shaped form on the outside under the ropes/straps if it distorts. Use plastic sheeting under the ropes and straps along the split to keep any foam that leaks out through the crack from adhering to the ropes or straps.

Prop the boat up so the Great Stuff will fall from the can's spout onto the part inside the hull where you want it to go. Do this in a location where you can leave the boat for a few days to be sure the foam has a chance to fully cure.

Empty one or more cans of Great Stuff ($3 each on sale at Ace) into the boat. One picture shows the location of this particular repair (port side, behind the seat), the other shows what the foam looks like after it solidified. Yes, it's ugly. But if you try to smooth out the foam before it sets up it collapses the foam and it is still ugly.

After a few days, put the boat back down on the ground and take off the ropes and straps. The foam will hold the split together. The can says that the foam should be painted, but years of experience suggests painting is unnecessary.