I like the Iberostar for family vacations. Dressel is definitely very inexpensive if you prepay and buy a weeks worth of 2 tanks...and Nitrox is free....and they fill their own tanks so you actually get 3100 psi...BUT...the diving is meh.
When I stay there I have one of the other dive ops pick me up notably Tres Pelicanos (3P).
When I stay in town, I stay at Casa Mexicana and again dive with 3P. Their shop is <5 min walk from the hotel. You meet there in the morning and they taxi you (at their cost) to and from the marina. They store and wash all your gear if you wish.
Definitely dive with one of the operations mentioned in this thread.
HOWEVER, I have been told by others that the island is super crowded in that holiday period and the taxi ride to the marina is very long. If I was going at that period, I'd avoid that completely, stay at the Iberostar, let my boyfriend enjoy the great resort and pool and either dive with dressel or have one of the ops mentioned here pick you up at the Iberostar.