Renting a boat is a fun and different thing to do at the MK area resorts. You can rent them from the Poly, Grand Floridian, Contemporary, Wilderness Lodge, or even Fort Wilderness. The resorts have a dock where you choose from little 2 person "Water Mouse" boats, mid size Montauk boats, or pontoon boats that seat up to 10 people. No matter which of these resorts you rent from, you can cross the water bridge between the seven seas lagoon and Bay Lake, which is neat because you actually go over the road to MK. There is some large boat traffic, but go with the flow of traffic and you'll be fine. You can boat around and see the beaches of the different resorts, and between the Wilderness Lodge and Fort Wilderness you can see the old River Country waterpark. Most of the lake area is open for you to boat on and explore, except for the North end of Bay Lake where they do the Sammy Duvall watersports, and this area is marked on the map they will give you.
We rented a pontoon boat with just two of us on our honeymoon trip to WDW and loved it. We can't wait to do it again when we go back in September. It is just a nice laid back contrast to a busy day in the parks.
