This article comes from a message board from Entilzha
This link is the entire thread. Below is an extract of the snorkeling guide
During our two-week stay in Baja Media Luna, I must have gone snorkeling in this bay over 20 time. Each dive lasted from 1h30 to 2h30 and I always tried to cover every section…although by the end, I was spending way more time in Turtle city, the coral islands and the Labyrinth.
Half Moon Bay is, in my opinion, the best snorkeling spot in the Riviera Maya. Water level is low so it is easy to see a lot. The coral is healthy and plentiful and we should make sure it stays that way. For those not staying in the bay, the startup is usually at La Buena Vida, located between the WASTELAND and the GORGON FOREST.
As shown in the map below, I have divided HMB into eleven sections. The number in each section indicates the number of turtle sightings.
THE WADING POOL : Water was 1-2 feet deep with rocky bottom with lot of seagrass and other algae. Visibility was from mediocre to nil. Fishes were mostly gobies although juveniles and the occasionnal trunkfishs could be spotted. The two turtle I saw in there were younsters (<1 ft) zipping around.
THE HIGH PLAINS : Mostly rocks, sand and silt with some algae and lots of urchins. Depth gradually increases up to the deep. A fair amount of waves since this section is not protected by reefs. Not many fishes but a good place to look for urchin shells. Visibility good to mediocre.
THE DEEP : Mostly silt with a few algae. Depth is 10-20 feet. Not much to see althouh I did meet my biggest turtle there (a 4.5 ft loggerhead). There is also the occasionnal barracuda. A good option to go from one end of the bay to the other since waves are not as strong as in the High Plains. Visibility usually good.
GORGON FOREST : Lots of soft arborescent coral and the occasionnal brain coral on a mostly sand/silt bottom. Low number of fishes but I mostly go there for the coral and the conch shells. Most of the shells already have inhabitants (hermit crabs) so don’t pick them up.
THE LABYRINTH : A very well deserved name trust me. Lots of hard coral and reef leaving corridors you can swim through. Extreme caution must be used so that the coral (and your skin) are left intact. Moderate amounts of fishes with large number hiding under the reef. Visibility is usually very good to good. One of my favorite spot.
THE SOUTH WALL : Impassable reef wall with a lot of waves. Stay away.
THE WASTELAND : Silt bottom with strong current. Nothing much to see even though visibility is usually very good.
TURTLE CITY : Name says it all. A relatively boring mixture of rocks, sand, silt and sea grass with a few coral islands. Visibility is usually passable to very poor. However, this is where the turtles hang out since over half of my turtle sightings (and all of my wife’s) were in this section. I almost always began and ended my dives in there.
CORAL ISLANDS : The junction between this section and Turtle City is a sudden rise in the sea bed leading to pretty rough waves. That is where squids and a good amount of fishes like to hang out. So if you can take the action, you won’t regret it. Just hang on to your mask. This section consists of islands of hard and soft coral where big brain corals are evenly interspaced for a thorouhly enjoyable snorkeling experience. Highest density of fishes is here including large schools of blue tangs and surgeonfishes. Visibility usually very good. My favorite spot overall.
SPONGE GARDEN : Mostly sandy bottom with some coral islands. Very good visibility. Most of the sandy patches are covered with low lying yellow coral or sponges (hence the name). Best spot for rays.
NORTH WALL : Impassable reef wall with heavy waves. Again, more prudent to stay away. However, I was able to explore it on a particularly calm day at high tide. Very beautiful. Saw three turtles in one visit.