The boat. They go to 5 diff. locations, one per trip out, and all are shallow and usually calm. If you have your own equip., bring it. The boat is a 45 min. trip and it takes aprox. 5 min. to get to most locations.
No snorkeling off the beach. You'll want to sign up for snorkeling at the water sports shed a day in advance but sometimes you can get on a boat leaving the same day if it's not full. They'll take you out a short distance and you'll snorkel around a reef. We had clear water and a lot of fish and things to see, but some folks told me when they went it was cloudy water and they didn't see as much. You can go again and again if you didn't see much your first time. Also sign up for the glass bottom boat tour. The boat captain will give you lots of info about the ship wrecks and local info. One other thing, sign up for the catamaran sunset cruise (booze cruise). Lots of fun on that too!
You will need to take the boat to see anything. CSA offers 3 snorkeling sites the best of which is the "Mexico" site. Make sure you ask them to take you here. Far more fish as well as an old anchor and cannon.
I bring my snorkel gear and each time I see some cool fish in front of the resort. Several starfish, a few stingrays, clown fish, tang, etc. Its hit or miss a lot though but I have fun. The snorkel trip is okay but the reefs aren't full of life.
We were at CSA last month and the reefs have bounced back quite nicely. We saw a lot of seagrass and lush green coral for the first time in years. Having said that, Jamaica is not the Great Barrier Reef. But there are plenty of fish to see on the snorkeling trips. Not much to see swimming near the beach since the sea bottom is nothing but fine white sand, so there is nothing to attract the fish.