We bring our own snorkel gear (first year just mask and snorkel so that we knew we had a mask that would work without leaking and a snorkel that 100 other people had not had in their mouths). We've done both the snorkel trail in the protected cove area (next to the family beach) as well as off the adult beach.
Snorkeling along the trail, you're guarunteed to see some fish as Disney has installed many fish-freindly objects for them to hind in/around. But visibility is not very good. There are lots of people in this area and barrier keeps it from getting swept clean by ocean currents.
Snorkeling off the adult beach has been great fun, but with mixed results. The first time we ever did it, it was one of the most fun things we ever did. The water was very still and clear. You could see things off in the distance and swim over to see what you would find. The place was littered with sand dollars, lots of star fish, and live chonch shells. I even had the thrill of seeing a shark in the distance.
The second time was soon after a hurricane had gone through the area and there just wasn't much to see in the open water.
The last time was during a period of rough seas and we soon realized that it was just too dangerous, and the rough surface was a killer on the visabiltiy.
Last time we when on a cruise, snorkeling tours were not yet offered on Disney's Island, so I can't speak to those.