I was just there two weeks ago and I dove with Tres Pelicanos for a very positive experience. I arrived the same way that you did, via Cancun, took the ADO bus from the airport to PDC, walked the three blocks from the bus terminal to the ferry terminal and arrived in cozumel three hours after landing in Cancun. it was very easy and inexpensive. There are many places close to Tres Pelicanos, my brother and I stayed at the Amaranto B&B and ate breakfast each morning two blocks away at the Cozumelena which opens at 7 am and we were always out within 15-20 minutes and just three blocks from the dive shop. Aldora is also an excellent dive shop and right across the street from Tres Pelicanos. We moved to PDC for our last two days and dived the cenotes with Cenote Xperience which was the highlight of our trip. We did 3 dives with them before flying out the next day. it was truly one of the most memorable dive experiences of my life and I LOVED that I didn't have to scrub all of the salt off my gear when I got home! I took $300 of pesos with me (over 6,000 pesos) and that paid for all my transportation and all meals for the trip. You get a better rate in both of these towns if you pay in pesos rather than the posted USD rate.