Aquaworst always have room for another diver. They have big boats, afternoon dives, and night dives. You might check comment on them.
I also dove with scuba cancun, boats are smaller. Their dive ladders are rusty and rough on the bottom, so be careful you don't get scraped.
As there is current, it is best to take a whistle or noise maker, and definitly a surface marker. If you do night dive, bring a small extra light.
Depending on where your hotel is, one might be closer than the other. Scuba cancun is on the north end of the 7, at KM 5. Aquaworld is on the south end - which means a much longer boat ride to reach your diving sites. The bus system works very well, and cost about 50 cents, and can get you and your dive gears where you wanted to go. I am not sure if either place have a shuttle service, you can google them and get more info.
If you do nitrox, you might be able to get 4 dives a day. With air, you could get a night dive in or two.
