I just returned from Akumal yesterday. It was our 4th year in a row staying and cavediving with Villa DeRosa. The Bahia Principe is just a blip down the road from DeRosa's. You questioned transportation from the airport....doesn't the Bahia have a bus or transfer van? A taxi will set you back about $90 US for the 90 minute trip. If you take a passenger bus from the aiport you will have to get off in Playa Del Carmen and get the next connection. This can be a pain with luggage, gear etc.
Yes, a cenote dive is a MUST. First choice for the open water diver would be Dos Ojos, second choice would be Grand Cenote. Any of the dive ops can set you up with guide for either. Be mindful of which operation you choose as there were two separate cave diver deaths this past spring as a result of contaminated air obtained from a "highly visual" op in the area. There are NO air quality regulations in Mexico and you're on your own....inquire about whether the compressor is electric or gas.
Zero Gravity is an excellent choice as is Gonzalo Vaccaluzzo at Aquatech. The Zero Gravity link was given earlier in this thread and the link for Aquatech is
www.cenotes.com.