For the last several years, I've come to Cozumel in December, and I love it. But there are pros and cons to all different times of the year. For several years, from 1994 to 2004, I owned a fixed-week timeshare at Plaza Las Glorias and I came in the last week of March every year. The diving was great, but the currents occasionally got a bit strange, and the water temps were a little cooler in March than in early December, only a couple of degrees, but I'm pretty cold tolerant, so It never bothered me. I liked it because of a Sports Bar called the Sports Page, which is now closed and gone - but in those days, I could always catch the NCAA March Madness Games at the Sports Page, and while Cozumel never got the Spring Break crowds that Cancun or Playa Del Carmen got, I could usually count on a good crowd of college kids cheering on their favorite teams. And The Sports Page served a pretty decent Grouper Sandwich with very tasty fries. I've also gone to Cozumel several times in July. Diving is good, currents predictable, water temps warm, and there is the option to snorkel with the Whale Sharks. But, when you are not in the water, it can be very hot and muggy, and rain is an almost daily event. Good AC in your hotel room and a decent pool (NOT the kiddie pool at Casa Mexicana) is a MUST for a trip in July, and prepare to get sweaty when walking to restaurants for meals. Overall, I think early December and late March- early April are my favorite times of the year in Coz, but a week in Coz at ANY time of year is better than working.
