Since I have only been to CSS, that's all I can speak to, so here's my input.
At CSS, you can walk out of the beachfront suites, across the lawn and to the main beach and then be snorkeling all in a matter of minutes. The main beach is in a cove, that has a coral reef at the end of it and is therefore well protected from large waves and boats. I think the snorkeling is very good, with lots of fish and sea urchins and starfish. The best areas to snorkel are out at the reef and along the cliffs on either side of the cove. Also, out in front of the mineral pool is good. the only downside is that the fresh water from the White River tends to sit on top of the salt water and can cause blurring when the two waters mix. if you keep moving slowly forward, this is not a problem. You can find many conch shells out near the reef and by the mineral pool, but keep in mind they are ALL home to hermit crabs and should not be removed from the seas.
i haven't been to CTI so can't comment on that, but neither of the Negril resorts have much to see snorkeling from the beach other than the occasional starfish or sting ray. we stayed at CSS last year, and i was able to snorkel from the beach and did see some cool things. either side of the cove had some coral with a decent variety of fish, an eel or two, and i think some sort of squid. i'm not an expert, so i could be wrong on what everything was i saw, but i was able to get my fix from the beach which i also enjoy and didn't have to worry about signing up for the organized trip.
We did the night snorkle at CSA and it was amazing. My husband isn't the most confident of swimmers, but he enjoyed every moment. As soon as we got in the water we saw a moray eel and we also saw rays, squid, octopus, basket starfish, sundowner parrot fish and of course sleeping fish. It cost $35, but was well worth it.
I would suggest CTI for snorkeling, especially if you have your own gear and like to venture out on your own. On our last trip, the last morning before anyone else was up, we snorkeled between the beach and the island. Saw some amazing creatures including a small cardinal fish and puffer fish. The water is usually very calm first thing in the morning and it's an easy swim to the island. Highly recommend it!
You can take the boat out with the group as well, but that's not always as rewarding IMHO.
I can only speak to the Negril side of the island but like already mentioned there isn't much to see if you just walkout. If you go on one of the snorkeling trips then it doesn't matter if you are staying at CN or CSA as they visit the same spots, and on that note if that's where you end up ask them to take you to the "Mexico" site with the sunken cannon.
We like to be able to snorkel from the beach, too. There is not much to see from the beach at CSA or CN as the sea bottom is mostly soft white sand with nothing much to attract the fish (such as rocks or sea grass).
CSS has the cliffs and lots of sea grass where the fish like to hang out. There is also an amazing amount of brain coral directly in front of the mineral pool/spa area. However, when we were there for 10 days, we could only snorkel from the beach on two of those days. The other days, the water was very cloudy and murky from silt and sand being mixed up with the water. The sight distance was less than a foot on those days.
The snorkel boat tours are all very good. It is a little bit limiting if you a snorkel fanatic like us.