Depends on the make of the kayak. If they are linear plasitc:
Perception, Dagger, Liguid logic, Recent Wave sport then
cut small sliver from the cockpick rim with a pocket knife
Tape some alluminum foil to the inside of the boat if you can
Heat area with heat gun until it is just barely shiney, not molten.
Heat plastic slivers at the same time by holding in front of heat gun as you heat the boat.
When both are shiney touch them together and lay sliver along the hole.
Push gently with finger covered in aluminum foil.
Repeated over heating will weaken the boat but it is not to hard. Practice a little on the cockpit rim to figure out temperature first.
the Heat gun needs a nozzle that focuses the heat. there are specific nozzles for plastic welding but you could just about do it with your crack pipe torch.
Crosslink boats - Duck tape repeatedly because you can not permenately fix.
Crosslink - Jackson, old Wave sport - up to EZ I think, Riot but Now they use linear.
Prijon I think you can weld but not sure. Pyranha I think is a form of linear as well.
friend of the fork, knife, and spoon