I have usually given 5 or 10 bucks when I have been on a boat trip. It usually depends on how good the service is they give. I have given more when there is only a few divers and a bigger crew and when there are "cheep" divers. These poor guys really bust their asses on these boats. Usually at the dock/shop by 6:30 at the latest for an 8:00 trip and then they have to unload all the used talks and then load all the fresh tanks, clean the boat, do whatever else needs to be done so they can be ready to go out for the afternoon trip. And then on wed and sat. nights they have night dives and they dont usually get paid well. Tip them well and they will do their best to make your trip great.