Yes, this should be done. It needs to be done. I neither have a pumpkin farm, or a kayak mold making company, but I support those that do in this endeavor. God speed brave pumpkin kayaker.
A plastic kayak would probably not survive the mold-making process, but that's fine, you can get a kayak either used (abused) from a local livery for dirt cheap depending on your bartering/scrounging skills, or a low-quality new one can be had for under $200 at a big box store, and simply be cut off the pumpkin.
I wonder if it would disqualify you to coat the pumpkin in a thick layer of Line-X:
I live right next to dills farm! There's recently been a controversy where protester have been trying to change how the water in a local lake is managed, and the lake have been completely shut down (as in it is now a dust bowl unless it's rained recently), so we can no longer have our yearly giant pumpkin boat races.
