Google "Dive With Martin". They have small boats, good DM's, and good rates and will pick you up at Iberstar's dock. The dive shop at Iberostar (Dive Palancar, if they are the same op) is not bad, but they are cattle boats. I would arrange a couple of shallow (less than 60 feet) dives first to get your bearings to get used to the current. The common practice on a two tank dive in Coz is a deep dive (80' to 100'), surface interval, then a shallow reef dive usually below 60'. Arranging shallow dives first with the dive op will not be a problem, especially if you book a package. If you are only diving in the afternoon or night then you may have to go with Dive Palancar, as the dive ops in town like Dive with Martin may not have enough booked to make it worth the trip to come pick you up. If you are traveling Funjet or Apple then the rep at the hotel can set all of this up for you, they do a pretty good job.
Not trying to schedule your vacation for you, but just trying to help. Have fun.