I tip a minimum of $10 per charter, so when my wife and I dive I tip $20 minimum. As sated, they normally work for tips only and they are there solely to make sure you have a good trip.
If the DM throws in little extras, letting us know about the dive site, maybe something special to check out on a wreck, whatever else I’ll tip $40-$50.
I’ve also found that if you dive with the same charter group on a regular basis and they know you and know you’re going to take care of them then they’ll go out of their way to take care of you too.