We did this excursion about five years ago.My wife and I were in our late 50s when we did it.
It is a scooter that you sit on and you put your head in a helmet with positive pressure.They take you down about 15-20 feet and you see the fish and turtles.Take an underwater camera for pictures.Our guide took our camera and after breaking up some crackers in front of our helmet the fish came to eat and he took a picture it was great.
They take about six people in a group and are down for about fifteen minutes.When you come up they take another group and you then have time to snorkle.
Try it and I hope you enjoy it.
When I did this excursion it was called "BOB", great time. Yes, you should have time for some shopping or something. They should tell you the length of time the total excursion is. As the previous poster mentioned, you have about 20 minutes or so on the scooter. The rest of the time, you can relax on the boat or snorkel until its your turn. They took you out aways another 20 to 30 minute boat ride.