halemano is spot on with what I understand to be acceptable (and tell my divers) when diving from a boat: $5/tank for an acceptable job, $10/tank if you had an "above average" time, and then scale up to whatever you feel like giving them if you had an exceptional time.
Of course, there is the option for less than $5 if you had to save a crew member's noob-a$$...
On Maui, most of the time, the diver guides are also the crew -- so tipping the boat is shared between everybody (captain, crew, dm's). I suspect it's easy enough to tell whether your provider is the same as the boat company to decide whether you need to tip twice or not.
Of course, there is the option for less than $5 if you had to save a crew member's noob-a$$...
On Maui, most of the time, the diver guides are also the crew -- so tipping the boat is shared between everybody (captain, crew, dm's). I suspect it's easy enough to tell whether your provider is the same as the boat company to decide whether you need to tip twice or not.