If you are looking to snorkel directly off the beach, CSS is the best since there is seagrass in some areas along with the rocky outcroppings of the cliffs where the fish like to hide. However, you need to bring your own snorkel gear to snorkel directly off the beach at any of the Couples resorts. Also, the water tends to get murky at CSS if there are waves crashing into the cliffs. During the 10 days when we were there, we only had good visibility to snorkel near shore on two days. The other 8 days, the water was too silty to see more than a foot ahead.
Both CSA and CN have soft wide sandy sea bottoms offshore, so you won't see much (although sometimes the stingrays have been known to swim close to shore).
The included snorkel trips are fairly similar at the resorts. We've taken the snorkel trips at CSA, CN, and CSS. Both CSA and CN go to the same reef area, although CN will also have trips within Bloody Bay. At CSS, there is a snorkel stop in deep water that is at the site of a sunken ship. That is definitely very cool and different.
Jamaica is not the Great Barrier Reef or even Belize or Grand Cayman. However, the snorkeling is perfectly OK. You will see a wide variety of fish, and the reef areas are fairly healthy.