Regarding snorkeling sites- I must have seen the reviews on RCI. Thought I'd go ahead and list the sites I compiled from the differing places:
AKUMAL BAY-Gradual sandy slope & no current-great for beginning snorkelers. Max. depth 12 feet.
HALF MOON BAY-Shallow rocky bottom, max depth 15ft, most doesn't exceed 6 ft, Great snorkeling! Access via La Lunita Restaurant in Hacienda de La Tortuga Condo's. Wear water shoes. Free snorkeling.
YAL KU LAGOON: Past half moon bay at the end of Akumal. Great snorkeling for all levels. No beach area, jump in from the rocky contour. Water averages 14 gt deep, but you can stand in most places. Snorkel toward the mouth of the lagoon to the right of the parking lot and see sting rays, sea turtles, shells in the sandy bottom. Inland, toward the entrance of the lagoon to the left of the parkng lot are plenty of bright colored tropical fish. Daily 8-6, entrance 60 pesos p/p.
PAAMUL: 16 miles south of PDC, closest beach to the hwy, easy walk from hwy stop. Crescent beach, calm water, great snorkeling. Beach club has the best cheladas!
XPU-HA: Between Puerto Aventuras & Akumal. Beach entry is just south of Puerto Aventuras, after Barcelo and before Robinson Club on the south end. XPU-HA Palace in on the north end, and the Copacabana is in the middle of the bay. An exquisitely beautiful beach!
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CENOTES(Riviera Maya)-reprinted from a review
CHAC MOOL (Claw of the Jaquar)-22 km south of Playa del Carmen. Almost across from Puerto Aventuras. 2 cenotes. Large cavern zone with beautiful views of jungle from inside cavern. Open 10-5 daily.
PONDEROSA (El Eden)-3 km south of Puerto Aventuras. Exceptionally beautiful. Short walk on path takes you to Coral Cenote, which has a large island in the center of it. Open 10-5. Nice overhang with tree that can be scaled to jump into cenote. One of the most popular snorkeling sites. Wide variety of fish, eels, turtles. Fee: $5 US
CHIKIN HA(Points of Direction)-5 km south of Puerto Aventuras just before Xpu-Ha/Across from Barcelo Maya Hotel/H11 on Riviera Maya Map/Long way down bump road. Footpath through jungle will take you to some other cenotes.
KANTUN CHI-just past Cikin Ha Cenote/H11 on Riviera Maya Map 4 mostly half dome cenotes along a series of light jungle trails. Most remote one has ancient looking Mayan temple next to it. Not very inviting waters. Fee $10 US
AZUL-Just past Cristalino Cenote/Close to highway/H11 on Riviera Maya Map. In light jungle setting with more open air/sunlit than Kantun Chi, which creates more algae. Fee: 30 pesos
TAJ MAHAL-26 km south of Playa del Carmen/5 km south of Puerto Aventuras/just south of XPU-HA- 4 interconnected cenotes. Requires swimming 5m underwater under a rock wall to come into a large open cave that has light shining through from ceiing above. Fee: 40 pesos
DOS OJOS (Hidden Worlds) 48 kim south of Playa del Carment/3 dm south of Xel-Ha/on right 4 km down dirt road/H25 on Riviera Maya Map. Part of Nohoch Nah Chich cave system. Location of Filming for IMAX Journey into Amazing Caves documentary. Open 10-5. Superb snorkleing, very popular. Stalagtites & Stalagmites everywhere. Fee: $10 US
TEMPLE OF DOOM (Calaver/Skull)- 2 km from Tulum on road to Coba on right/Very rugged rocky unmarked jungle path approximately 50 yards from road.
GRAN(Sac Aktun/White Water)- 5 km from Tulum on road to Coba on right/V1 on Riviera Maya Map. Ladder steps lead to half moon shaped cenote decorated with small passages & openings. One of the most popular sites. Good for all ages. Shallow on one side/deep on other. Famous for brilliant speleothem decorations and crystal clear water. Open 10-5 daily. Fee-50 pesos. Hope this helps with your planning...