I dove with ScubaDu 2 weeks ago for 3 days. I thought they were very good, all the dive master and the boat crew were pretty cool and very helpful. They will help you setup and switch tanks if needed and give you a hot towel at the end of the day.
For this dive shop, I would recommend you tip them at the end of each day. I usually tip at the end of my trip since I dont like to carry cash with me. We got new crew each day I was there and I had to chase some of them down to tip them when I was leaving.
One thing I dont like about diving in Cozumel is how early you have to get up to go diving. My days were like this:
6:45AM wake up.
7:00AM Breakfast
7:45AM Be at the dock.
9-10AM Dive.
I was staying at the Presidente and I was told some dive shops further north depart even earlier.
It did not feel like a vacation waking up earlier than when I go to work.