They are offered at many of the Disney resorts that are located on a lake. On Bay Lake, the Contemporary, Wilderness Lodge, and Fort Wilderness offer them.
You can also connect to Bay Lake from the Seven Seas lagoon if you rent from The Poly or Grand Floridian, which have them as well.
On a different lake, Port Orleans, Old Key West and Downtown Disney offer them on the same lake, and they are also available at Yacht Club / Beach Club, and Coronado Springs on seperate lakes.
Here is a link to another site that has the picture of the rental sign at Wilderness Lodge, which shows the hours at the time and the cost, which was $32 for 30 minutes of $45 for an hour. Usually they are open from the early afternoon until early evening, but not after dark. The pricing is probably the same at all the locations.
I prefer to rent on Bay Lake because there is a lot to explore, and you can see the old Discovery Island and River Country waterpark and cross the water bridge to the Seven Seas Lagoon in front of the Magic Kingdom.