Probably too late cuz you're probably already there, but for anyone else thinking of going to Riviera Maya:
Stay at one of the cheaper all-inclusives in Cancun. Eat your breakfast there, travel down to Tulum or Playa del Carmen or Puerto Aventuras to dive and then come back to your all-inclusive dinner, drinks, etc. each night. Here's how. Day 1 - take your dive gear on the Cancun city bus (less than a US dollar) to downtown Cancun to the stop nearest the Collectivo. Then take the Collectivo down to whichever town your dive guide wants to meet you in. The Collectivo is a van service that basically charges by the seat for however far you want to ride it down the highway. It's about $2 US. The first day you will have all your gear, so on the way down you'll want to buy 1 or 2 extra seats for your gear so you don't have to hold it all on your lap as the van fills up with more passengers. Then, after your dives, leave your gear with your dive guide for the next day. On the way back you'll only need one seat on the Collectivo. total transit cost $10.
Day 2 - Bus, Collectivo, dive, dive, Collectivo, bus, eat, drink, be merry. Transit cost $6
Day 3 through N-1 - repeat Day 2 (preferably at different cenotes). Transit cost $6 per day
Day N (last dive day) - Bus, Collectivo, dive, dive, gather all your gear, pack up, tip your guide, buy a couple extra seats for your gear, Collectivo, bus, hang your gear to dry, relax. Transit cost $10.
Day N+1 - enjoy your hotel's amenities and pack for your trip home.
This plan also works great if you have a non-diver spouse, family, or other travel companions because they will remain entertained, safe, fed, watered, and happy at the resort while you are off in the jungle diving, and you still get to spend significant time together every evening.