Thanks for everyones reply. We shuffle between a small travel trailer, and the boat. The boat stays in the garage during the winter, and goes to AZ during the summer. Probably a total of 5 times a year going over the lawn. However, our current boat barely fits in the garage as it is, and we have been looking at buying a different boat sometime in the future.
After seeing Sethro's photo, I'm going to pursue a garden tractor. I would much rather have a power mover, but I'm not sure it'll work over the large river rocks. Thanks everyone for the ideas!