Ah-yeah, the grand ole tipping debate. If you have to ask, you can't afford it! I love it when people proclaim that they "ALWAYS tip AT LEAST $x". We'll, I ALWAYS tip AT LEAST $0! If the service is bad, they get nada and it goes up from there.
Tipping is a personal decision. The standards (15% to servers, $5 per tank, etc.) only exits for less civilized people that have a habit of tipping. The standards are a reminder to them that tipping is customary since the person(s) receiving the tip are more than likely being paid peanuts. If you go on what feels like a $2-tip dive, tip $2. If you go on a $10-tip dive, tip $10. But please, don't do the all time injustice of always tipping $5 regardless of anything else.
With that said, I've had very few instances of a $2-tip dive or less in Cozumel.