Everyone is makes renting boats waaaaay too complicated. Here's the deal--
ALL of the marinas rent Sea Raycers by Sea Ray boats. There's no difference between one resort and another. The water mice are gone.
In order to rent a Sea Raycer, you must be 18 and present a WDW resort ID or valid drivers license. In order to drive a Sea Raycer you must be at least 12 years old and a minimum of 5' tall. At the MK resorts there has to be someone with a valid drivers license in the boat--they don't have to drive, just be in the boat. Max. 2 people and/or 320 pounds in the boat.
You can boil it down to...mom or dad has to rent the boat, your 12 yr old can go by himself/herself, unless you are at MK, then you need to go with.