Someone with a lot more familiarity with the area will probably post a better reply later
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But for my 2 cents, since I am also going diving in Cozumel in August, I would suggest researching the numerous dive ops before you go and pick what you want. The prices and boats/equipment offered vary some. Also not all will pick up from all hotels so factor that into your decision.
I did a lot of looking before picking one. There are many many operators and everyone seems to have their favorite. I don't think I saw anything really bad about any of them, so it seems like personal choice on that one.
I would contact them and get your dive plans locked in so you don't find them booked up and have to go with your second choice after you get there.
Depending on where you are staying, if you just plan to go with the resorts dive op you would probably be safe to just walk up.
It is also going to depend on the size of your group. If it's just you, or you and a buddy you can probably book last minute. A lot of the better dive operators advertise the fact their boat groups only consist of 6-8 divers. So if there is more than a pair of you it might be more difficult to book last minute on a preferred smaller group. If you've ever dove small group vs a big "cattle boat", the smaller group is better.
Maybe I am just too much of a planner, but we booked in April or May for August and sent the deposit to the dive master the same week.