My wife and I, as well as our group of friends have been diving with Edmundo for about 12 years, at least once a year. We just came back from Cozumel 3 weeks ago and dived with him for 7 days. Chachacha is a small shop in an alleyway right around the corner from the Casa Mexicana hotel next to two jewelry and souvenir shops. Edmundo and his family live in a large apartment over the shops, which he owns and rents out.
Edmundo is somewhat of a local fixture on Cozumel, having been in the diving community there for more than 40 years. He knows the reefs like the back of his hand from Barracuda to Punta Sur. He has one fast boat with 2 brand new 60hp outboards. It will comfortably carry 7 divers. Edmundo does the guiding himself, and has a steady boat captain who, of course, stays with the boat.
You can pick where you want to go and how you want to dive, although he does politely and unobtrusively assess your skills before doing the deep hard dives.
The operation is pretty informal. I don't know what he does for other people, but he has never asked for a deposit, or insisted we pay if we can't make a dive. I think you will find that his prices are among the more reasonable on the island.
Check out Chachacha's many reports on Trip Advisor. Most of them are 5 star and his overall rating is 5 star. There are a couple of whiners there, but you will probably will be able to see through them.
Give me a message if I can answer any other questions. Obviously, I love the guy.