Another diving option, both boat and shore, would be Scuba Shack, which is basically right across the street from Casa del Mar but about 50 - 100 feet south. Bright Yellow stand. They have lockers you can use if you dive with them. Bring a paddle-lock. You probably don't need one, but better safe than sorry. I did 10 or so tanks with them on my last trip and really enjoyed the operation. Roberta was sending some kind of bundt cake for the surface interval between shore dives that was crazy good. Another great Cozumel dive op in my opinion. I haven't experienced diving with Del Mar Aquatics. What I've seen is bigger boats, but I'm not well acquainted with the operation. Scuba Shack was small boats. Diving with Scuba Shack avoids the gear lugging that Don mentioned above. I know exactly what he's talking about, and it's doable, but somewhat more painful the older you get. Even though I've been on the top floor (3) of Casa del Mar more often than not, and literally the furthest you can get from the exit, it hasn't killed me, and probably won't kill you if you stay at CDM and dive with an op other than Del Mar Aquatics or Scuba Shack. Many shops will take care of your gear for you, but even then you wind up wanting mask, fins, snorkel, wetsuit for snorkeling, and if you plan on doing some afternoon or night shore dives, well, you'll be lugging gear. Diving with Scuba Shack avoids that completely.
You can get tanks and weights from Scuba Shack for shore diving even if you don't dive with them, and they're probably about $6. They may even be free if you dive with the shop. I did that a couple times, but don't recall if we paid or not. I'm pretty sure they were free. We entered and exited via a ladder into a protected little cut-out in the rock. Probably not as easy as the entry right next door at Del Mar Aquatics, but easy enough. I did one little shore dive wherein I rented the tank and weights from Del Mar, and the Del Mar guys were very nice. Also, if you're staying at Casa del Mar, they don't charge to use their pier. Though there are other places I'd like to stay in Cozumel, Casa del Mar is just fine, really great in many ways, and I continue to stay there in support of their pier use policy, among other reasons. So far, at least, they seem to "get it", when it comes to the pier use controversy, and as a result, they get my business when it comes to hotel stay. The staff at CDM is very, very nice in my experience. The food may not be 5 star, but there are many great dishes. Try the queso fundido. (sp?)
Oh, and I don't know what it is, but I have to have a club sandwich when I'm there. Don't know what it is, but it rocks. I'm kind of a picky eater, but haven't had anything at their restaurant that I didn't like. I am, however, a big fan of all the great restaurants in Coz, so I eat most suppers in town. I've had many satisfying suppers at Casa del Mar, but for the most part, I eat in town at night.
When we shore dove from Scuba Shack, we entered at the shop and veered toward the Del Mar pier, and followed the drop-off. It sort of curves around, but you're heading north, basically. Did a night dive there too. On all dives, saw lots of stuff. Very nice for pictures. Be aware, though, that the profile I just described has you swimming *into* the current when you return, so plan your air and stamina accordingly. Also, there's some boat traffic, so keep that in mind too.
My dive op of choice is Deep Blue, and I will continue to dive with them when logistics permit, which has been almost always, save for my last trip in March of this year. I say this so as not to have anyone thinking I'm a ravenous Scuba Shack/Blue Bubble/Roberta's Eco-Dive (basically all the same op) fan. I'm a huge Cozumel Dive Industry fan, and logistics allowed me to experience this dive op, which did a great job and sounds like it might be what you're looking for, thus I post.
Hope this helps. If you have any questions, fling 'em at me.
-Blair