Tres Pelicanos gets recommended here quite a bit (and has been my dive op the past 4 years)... 8 minute walk to 3P's shop from the ferry pier. Small boats, Scubapro equipment maintained by the shop manager who is a Scubapro certified tech. AM dives depart the shop at 7:40 AM (courtesy transport to the boats and back is provided) and afternoon dives depart around 12:30PM I believe? Perhaps someone can comment on the time the afternoon transport to the boats leave the shop. Contact Jeanie through Tres Pelicanos' website with your schedule and she'll immediately respond whether or not your schedule will work with them, if they have availability, and how the boats are looking for afternoon dives. Depending on the time of year, afternoon boats aren't always going out if there isn't the demand for them.
Scuba Diving Cozumel -Dive Packages-Top Rated Cozumel Dive Shop
Just a thought, if you are traveling solo it may be nice to book a room in town on the island for the night before your dives. Coz is a very different experience from PDC and is definitely worth spending some more time time topside than just viewing it as a day dive excursion from PDC. You can find great rooms at great hotels in August for around $45 taxes included and depending on where you stay that will include a light breakfast to a massive breakfast buffet (thinking Suites Colonial or Bahia Suites). Come over the day before, have some time to walk around, eat, drink and relax. Pack up the next AM and leave your travel bags at the dive shop while you go diving. Back to PDC with your travel bag. Easy, relaxed, see/experience more and not be rushed for an extra $45 for a room for the night? That's an easy decision in my book.
If you tend to hang shallower like I do, no need for Nitrox as most all dives offer multi level, gradually ascending dive profiles and by hanging shallower. I never hit ND limits or run low on air before the deeper divers on Nitrox do and I almost ALWAYS see 1HR+ bottom times with 3P's.
I agree with ReefHound, Aldora get's its share of recommendations here and is also a top rated op but after owner Dave's rant about cruse ship divers and his subsequent refusal to serve them I'm not sure you want to consider that op. I mean, the ferry you are taking may as well be a cruise ship as you aren't on the island the afternoon before or waking up on the island the day of your diving.