We enjoy renting boats, especially on Bay Lake (which is where WL and FW are located). You can actually rent from either resort, and the boat options and prices are the same. You can cruise around Bay Lake, go around the old Discovery Island, and cross the water bridge into the Seven Seas Lagoon where the Grand Floridian and the Poly are located. It is fun to drive the boat right in front of the Magic Kingdom.
You have some options as far as what size boat to rent. We usually rent a pontoon boat, which holds up to 10 people, although we had a ball with just two of us on one. It is relaxing to have seats for everybody as you cruise around and enjoy the view. As of last summer, the cost for a pontoon was $45 per 30 minutes. Here is a picture of the pontoon boats:
You can also rent a smaller Montauk boat that holds up to 6 people for the same price. These look more like a regular boat than the "party barge" pontoons do, and might be a little faster, but no WDW rental boats are what you would really consider "fast".
Another popular option that a lot of people enjoy is the small "SeaRaycer" boats that seat up to two people. These cost $32 for 30 min or $45 for an hour, but you might want to rent more than one to have your family all go out together. They are small and a lot of fun, but you will need multiple drivers.
Overall, we definitely recommend renting a boat and exploring Bay Lake. Below is a picture of the rental rates sign from last September when we were there:
Let me know if you have any more questions, and enjoy your trip!