When I did my referral dives for my PADI Open Water, the dive shop that was used was Yukatek Divers. The dive centre is located at Avenida 15 norte, between Calle 2 and 4, Centro. I would recommend this shop to anybody who is going to visit the area. The owner, Jean-Yves, is from Switzerland. The dive shop's website is http://www.yucatek-divers.com