Every time I've been to this village the SCUBA operation has been set up differently. On my last visit they had one dive boat that did one to three trips per day, depending on the day, all single tank dives. During the week the trips were: M-F 8:45A & 10:45A; also T,Th,F 1:45P. Sat 8:45A only and Sun 10:45A only. Wed is the night dive at 6:30P. The 10:45 was ALWAYS full. If you want a comfortable dive, get up early and do the 8:45. The 8:45 usually goes to the deeper sites, but you'll never really never find anything deeper than 90 foot around Cancun. The Sat dive is special and goes (weather permitting) up the coast for a dive off Isla Mujeres, returning around 12:30P.
A dive computer is overkill here unless you are taking the excursion to Cozumel. Do bring some kind of timing device. That may sound stupid, but I've seen many that forgot; they will not let you dive without one. The Chief of Scuba even resorted to renting watches.