Medwyn Williams, 67 of Llanfairpwll, Anglesey, is growing a 725kg squash with 15cm thick walls for the journey, Metro reports.
Williams said: "I am totally confident this can be done - we shall call her HMV Cinderella. Her Majesty's Vegetable.
"I have done tests with smaller pumpkins and they float well. I might just make it to the Isle of Man."
Medwyn hopes to fit a motor and seat to the pumpkin for the journey in aid of Help for Heroes. He will be joined on the trip by his 42-year-old son Alwyn, a former soldier who served in Bosnia, Northern Ireland and Iraq.
The world record for the largest pumpkin was set last October in Ohio at 782kg.