We did this excursion in April. You take the shuttle to the site. You are in about mid calf deep water. It's pretty good, but several years ago we did the dolphin excursion in Nassau and it was much better.
What Ltuttle described is called dolphin encounter. Swim with dolphins is a lot more fun. There is also also a nice jungle garden there and a beach. The beach is pretty rocky so take some water shoes if you want to swim in the sea.
Swim with the dolphin is in deep water, at least the time my daughters went in Ocho Rios it was. They had a great time. After the swim we could hang around the beach and gardens and then they took you back to the ship.