I drove dive and passenger vessels in Hawaii for a lotta years - I never got tipped. I was paid pretty well - way more than the divers or mates. Don't worry about tipping the captains UNLESS you feel like it.
When I worked the diver end of it, we never got tipped much either. But we were all paid. Some guys hustled for tips, but we had special words for guys like that. Some of the boats had prominent tip jars, but that seemd tacky.
When I ran my own business, my DM's were paid pretty well - plus it was all cash for them to make my accounting life easier. For some reason though, we were tipped unmercifully - it was never solicited, never expected, and I always made sure that the clients knew that they were horribly overpaying me, just to make sure that there was no mistake there. So that was ok.
When I got to Florida, I found that DMs were not paid - that they worked for tips. I still wonder about people doing this for free - a lot of them do free maintenance and repair work as well. So ya kinda have to tip them.
Don't get hung up on the amounts. Tip the DM if you feel like it unless they pretty much do nothing onboard except dive and you don't feel like tipping them. If you need help with a quick repair, a dive guide, help with your pricey camera gear, you better slot that person some extra shekels for the service(s).
If there's a DM onboard to meet the USCG crew requirement for an inspected vessel, then that person should be getting some upfront dough from the owners. This unpaid DM stuff is a long Florida tradition, and I still shake my head at the wonders of voluntary involuntary servitude.
But there seems to be a neverending number of people wanting to work in the glamorous world of professional dving in southern Florida.