Snorkeling Tres Cocos is pretty interesting. This spot is often combined with Mexico Rocks into one trip by the snorkel boat companies based out of Ambergris Caye in Belize.
Unlike Mexico Rocks, which is between the barrier reef and Ambergris, Tres Cocos is actually on the inside of the barrier reef.
It is very shallow snorkeling, probably averaging only four to eight feet deep. We saw many more fish here than at Mexico Rocks, and there was lots of Elkhorn Coral and large Brain Coral.
There was a decent amount of ocean current at this location. In fact, the boat dropped us off at the north end and we swam with the current towards the south. Then, the boat came and picked us up so that we did not have to fight the current back. We have often found that you can experience stronger currents in shallow areas like when snorkeling Tres Cocos.
Here are a few more pictures of what it was like.
We found a nice variety of soft corals and fans, and a mix of tropical fish, although not many larger species.
Tres Cocos is so close to San Pedro that this is a pretty quick boat trip, and well worth doing. It is easy to recommend.
Caribbean Fish, Creatures & Corals ID Books