I've done Cozumel solo many times in the past - everything from long weekends when I found a dirt cheap flight (a lot rarer lately) to a week or even 10 days. My usual routine back then was to just fly down without any reservations, and check out my favorite cheap hotels downtown. I never had a problem finding a bed, but then I wasn't looking for fancy accomodations. I would then dive with Aldora, small boats, everyone dives computers, and HP Steel tanks - either a Steel 100 or even a Steel 120 (my option, but I'm fairly large; if you are smaller than 6' or skinny, it can be pretty heavy), so lots of bottom time. Aldora does cost more than other dive operators, but considering the service and extra bottom time, I always thought it was well worth it. If you are looking to save money, you can find cheap accomodations, and you can eat very well at a very reasonable cost at the locally- owned places. A great meal at Sabores, a loncheria (only open for lunch) will cost you 70 pesos (about $5.50) plus tip. Lots of local options back in town where you can eat cheap and well.