I asked the same question before my gig with Dressel Divers in early August. I also got mostly negative statements. Needless to say, I was really concerned.
After diving with these people for a week I can empathically tell you that they were great. Yes they have large boats. Two instructors (no dive masters --- all their dive guides are instructors) per boat, and no more than 6 to 8 divers per instructor. One day we had the entire big boat to 7 divers. Their entire group of instructors/captains/tenders were the most congenial that I have ever had the pleasure to dive.
They are very safety conscience. They limit most dives to 80' and one hour bottom time. They deploy an underwater surface marker on a reel so divers can ascend reducing the likelihood of a boating mishap.
If you are looking for a more risk taking adventure, you might be happier with another operator. If you are interested in safety, Dressel Divers is your answer.
If you are renting equipment, Cozumel is not recognized as the quality scuba gear rental Capital of the Caribbean. Dressel divers had the best organized and best looking rental gear that I have seen with includes the States.
Located on the Southern portion of the island and very close to the most of the great dive sites, Dressel Divers returned to the Iberostar after every dive. They scheduled and published the dives for the entire week. One could choose to dive four times a day, two dives in the AM, two dives in the PM, one dive in the AM, and one dive in the PM. I really liked the dive mix n match flexibility.
The combination of the Iberostar and Dressel Divers resulted in the best dive vacation my wife and I have had to date.