How many of your group are diving? Small ops are limited to 6 or 8 in many instances due to boat size.
In house ops can be convenient but aren't necessarily cheaper, and often have a schedule meaning that even though all the divers still have air you will be forced to surface to match the scheduled time that the boat must be back.
Re the all inclusives, the general ranking of those you should consider is Cozumel Palace #
1, in town & the best AI on the island. Wyndham Aura, NEW, little is known right now but it should be a high end resort from all the advertising & hype, and although not in town isn't too far away and should be free of kids. IF it lives up to it's billing it may very well become # 2 but that's yet to be seen. The Occidental Grand is currently # 2 but far from town, BUT very close to the dive sites & has a good in house dive op. Next is the Iberostar, further from town, good in house dive op that I personally have used & feel are a bit overpriced. Great resort, good food, well run, but a long trip to town by taxi. (I haven't stayed at the Grand so can't compare but have dove with their dive op). Next is Hotel Cozumel, which is in town, although at the southern edge of it as such. Nice rooms & property, decent food but by no means great on a daily basis, but in most cases you can get a really good deal on the AI option that makes it a bargain AND allows eating out without feeling that you are paying for 2 meals at the same time.
The Park Royal is still hard to rate due to the mixed reviews, but from all of them I have read it falls short of the star rating it has given itself, but it most likely is higher than Hotel Cozumel but only marginally.
I have photos of the grounds at the Hotel Cozumel. the Park Royal, the Iberostar, & the waterfront section of the Palace here
Picasa Web Albums - Larry
As for the Scuba Club it is a bit unique in that it is an AI but a rather small one with diving being it's primary attribute, and although my knowledge re it is only from reading about it it may suit some of your group but I doubt that it will meet all of their expectations. It is close to town (across from Hotel Coz & a tiny bit north) but not really my idea of an AI resort. There have been a few very good reviews here relating to stays & dive trips from it.