Renting the boats is one of the must do for me family. We using end up renting them 3 to 4 days during a typical stay. They changed the rules for drivers in the past 2 years. Used to be you had to be 12 but they had several "bad" accidents with the young careless drivers so they changed the requirements that you have to have a drivers licence to rent. My DD rented when she was 16. Two can ride with a weght limit of 300 lbs. You don't want much more then that anyway because you would not fit and the boast would not go very fast.
As for the cost, go by the posted rate but I have not figured out how this works. We are never charged the amount that I think it would be, it is always less. There have been time we are out for 45 to 50 min but they charge for 30.
You definately want to rent at a MK resort. You have Seven Seas and Bay Lake to run them. There is alot to see and the waves are much better with the other boats on the water. Jumping the waves is a blast.