Hello,
The best places to rent a water mouse are the Polynesian, or the Contemporary. You do not have to be a resort guest to do so, and unless all boats are taken, you don't need to make a reservation. You could always call from Guest Relations at whatever park you are in before heading over. The reason I like those resorts best for a water mouse, is that you have the largest area to boat on, between Bay Lake & Seven Seas Lagoon, and the marinas are easily accesible. The areas available at places such as Y&BC, CSR, etc, are much smaller than what is open at the MK area resorts.
My 14 year old daughter was allowed to drive her own boat this past fall, I believe the cut-off age is 12, but you should call and make sure.
Hope that helps,
Take care,
Bill