WINDSOR, Nova Scotia -- Leo Swinimer carefully climbed into his boat and dipped his paddle into the pond last Saturday afternoon. The craft rocked back and veered to one side. Keeping the 600-pound pumpkin on course was going to be a trick.

"It's a cranky one this year," the 72-year-old said, fidgeting in the hollowed gourd as he tried to get his balance. Mr. Swinimer then attacked the pumpkin with a drywall saw, enlarging the opening enough to give him more room to paddle, but not enough to flood the boat. "If you don't sit...