I always give 15-20% depending on the quality of service. You have to keep in mind that usually the only person getting paid is the captain. The others work on tips alone. So, if they take care of you, take care of them.
Also, all of the local dive boats I’ve been on (1,2 or 3 tanks dive charters) I give each of the crew their share. If it’s a liveaboard, usually you can give it to one person - the Dive Master. They will divide the tips to the other crew.
If you are unsure, just ask the captain.