We used Capt. Marvins in Grand Caymans for a sting ray tour. We had a great time and I'd use them again. We booked directly and I don't think it was much difference in price from booking through Disney. 2 things to help you decide. In Grand Cayman you need to use tender boats to get ashore. If you book a very early excursion directly, Disney sends their excursions off the ship first and you could be late to your excursion, and Disney can decide to cancel excursions if they like and offer you a refund, we booked directly and because of rough seas Disney cancelled all morning excursions, when we finally went ashore Capt. Marvins was only too happy to take us on a later excursion.