We did this on our own-very easy. Cost is $10 cab ride for four each way. There are cabs waiting in a line when you exit the dock. Ignore the vendors trying to sell you tours. Admission to the park was $10 each at the gate, children 7 and under free, but they did not charge for ds- even though we told them he was 8. There is a small wading beach, but the snorkeling area was where you walked down some steps carved into the seawall. Go early because the ship excursions will show up around 1- too many people will scare away the fish. The day we were there, there were 6 ships in port. We have our own snorkeling equip-well worth it if you are planning to snorkel more than twice, and you know that your mask will fit-but, you can rent equipment at one of the dive shop stands at Chankanaab. The dolphin show is extra and the dolphin swim is a lot extra. There are restaurants/snack bars to grab a snack or beverages. And you can explore the jungle on paths and see iguanas running around and historical stuff throughout.
When you are ready to go back, there will be plenty of cabs lined up at the entrance. This is so easy to do on your own that I would not even think of paying extra to be with a large crowd and be told how long I could spend there. Have fun