This idea of cutting a Kayak to size seems a bit dumb. Why don't you
just build a bigger storage space? Or if you want to work on a kayak
buy a stitch and glue kit for a smaller Yak. But if you insist on
cutting the kayak. Perhaps put in a 2 sturdy bulkheads about a 1 mm (or
less) apart and cut between them creating two water tight sections that
you can reconnect by butting the bulkheads together and bolting them
firmly in place. You might even want to have 3 reattach-able sections
stern cockpit and bow.