http://www.cancuncruisingadventures.com/monohulls.htm site is down now but you could check them out when you get down there
http://www.sailingcancun.com/adventure.html Here is the other charter company down there.
Mexico from my last visit doesn't allow bareboat charters. You have to charter with the crew captain. Both times I have charters the captains have been great. Older boats but in good working order. Prices are reasonable.
These are the only two companies that I know of. I haven't been down there in two years, so unaware of any new companies.
Melissa