We rented wave runners in Key West and Grand Cayman. In Key West we used Key Kat Marine, it was down a few blocks from the dock. They have a 2hr guided island tour that was totally awesome! DH had our 5yo DD and I had our 9yo DS on our wave runners. The guide takes you all around the island and stops in a few spots and gives you a little history of the island. We had a great time.
In Cayman we rented from one of the beach side places. It was fun but you could just take it out a little way and just run around, not much to see although it was still fun.
Good luck on whatever you choose.