If you are staying on the cliffs, most likely you will see one of Vincent's boats go by 2 or 3 times during the day, starting around 9AM. He has been around for over 25 years and I have used him a bunch of times over the years. Some of what he does---he can take you out to the reef for snorkeling (about a 20 minute boat trip out), he can take you on a cliffside sightseeing trip, which I did this last year and got some great shots of cliffside resorts, and he can also take you to the Royal Palm Nature Park up the river from the roundabout in town--this place has the endangered Royal Palm tree , birds, alligators and trails to walk through the bush (make sure and wear mosquito repellant!!) Vincent is a very safe, easy-going guy who charges around $25-$40 per trip depending on where you want him to go. He does have more than one boat with assistants on the others, but they all have Vincent's name on the boat. When you see him, just wave him in and tell him what you want to do...Some resorts are easier to get in his boat from the cliffs than others-----
You can get into the ocean at any of the cliffside resorts, and some places have better snorkling than others....I can definitely recommend Catcha Falling Star, Banana Shout, and Seagrape Villas as good snorkling spots so long as the water is calm-----Morning is usually the best time for snorkling, or either around 4 in the afternoon---the sun is pretty merciless in the middle of the day. I recommend taking your own equipment---most large stores (Target) have snorkel stuff pretty cheap, and then you can leave them with a favorite Jamaican for a tip or in trade for something, and have a little extra room in your luggage for rum! Also, as indicated by the photo here, cheap disposable underwater cameras sometimes take really good shots!
What a time we had! Because it was off season, we were the only 2 people on the ship--essentially a private cruise. Went out in to open water, snorkeled the Montego Bay Marine park, went to Dr. Cave Beach, Mo Bay Margaritaville, and then back to the pier. The three staff members were very nice and great fun to talk to. VERY high quality snorkel equipment and a snorkel guide goes into the water with you. The ship is the one they used in the movie Treasure Island--way cool! Open bar didnt hurt, either. ha ha ha
The best thing I did in Jamaica was a snorkel trip. It was a day trip on a catamaran that took us out to a great reef for snorkeling. The trip included lunch (at a stop on a local beach) and drinks while on board. The trip back was great, we even saw a dolphin swimming next to us!
Most trip packages will offer these kind of trips or you can find out more at your hotel.
Captain Moses' Glass-Bottom Boat Ride is so fun mostly because the staff makes it fun, by dancing, singing, climbing the cliffs and jumping off and taking you to the caves to snorkel. They will take you anywhere from the Island offshore from Negril, to Rick's Cafe so that you can watch cliff diving up close. They are very safe and reliable in terms of time. They appreciate making new friends more than haggling over prices and forcing you to choose their business over others. Very friendly and outgoing people.
While I didn't see anything terribly exotic, we did spot a few stingrays near our resort in very shallow water within easy reach from the beach at the end of the resort (currently the clothing optional beach). I also saw an eel and the usual variety of tropical fish you find in the Caribbean.
Our resort also had a snorkeling cruise, they change the snorkeling spot based on the water conditions, the day before they were taking people over to the 7 mile beach side, the day we went we stayed inside Bloody Bay where there is a decent size reef with plenty of fish.