The contestants scoop out the giant pumpkins, paint them festively, get inside and paddle them across a lake! I couldn't find pictures of this year's event but the link will give you the general idea.
Pumpkins float very well and are reasonable sea-worthy but not easy to paddle. I think there is a movement afloat (ha-ha) to breed a pumpkin with a pointy end to cut the water better. But you didn't hear this from me!