So I am a man, but with a young daughter (extremely attractive, that's not Papa's bias either). How would my daughter deal with this situation? While she's only 5, if her behavior from when she was in daycare is any indicator, she'd knee the guy in the nuts so hard his testicles fly out of his ears, legal consequences or not (which would really escalate things in a bad way for that person). She will be learning MMA and sometimes I teach her some boxing.
Definitely always speak up, and speak up loudly. Make sure as many people can hear you. If your instructor or dive center does not back you up, get out of there. It just isn't worth it. There's a lot of shiitty aspects of the dive industry and allowing customers treat female staff that way does happen, but it is never acceptable.
I would also go to the local police station and speak to someone about the next time, as there unfortunately will likely be a next time, to find out where is the line where there are no legal repercussions for you. So when it happens, you can go right up to that edge.
As Mr. In-Between says, there reason there are so many аsshοles is because they are allowed to get away with it.