Only as much as you feel the crew earned.

Same with food service.

In America, waitpersons get taxed at 8% of the total they served, as if
everyone they served tipped 8% of the total tab, which generally doesnt
happen, which is why they get zero dollar paychecks. Some deserve no tip,
some deserve to be tipped whatever you can swing.

Myself, I tip $5 to the barmaid for the first round, no matter what. If the
drinks are good, and she keeps track of us, she can expect that *at least*
at every stop to our table, plus a nudge at the end. If the drinks suck, and
we have to wait too long, nothing, and we leave.

Same with dive ops. If they are good, let them know it, pop them a decent
tip (20 bucks) and you will be treated like royalty.

If they are a.sholes, shun them, no tip, and bitch on the lists.

Depends on the service

Locally -

The nursing student who always scores the seat right by the ladder for
me and my doubles, who is masterful at cat-herding newbies and their
gear out of my way, who brings me hot soup as soon as I am out of my
gear, who pours hot water into my hood before I don it for the second
dive, gets $20.  The other DM kid who learned the routine from her gets
the same.  The kid who does the basics minus the cat-herding and soup
service gets $10.

Depends on where you are and what kind of service you get.  I tend to
tip higher in places like Hawai'i because the cost of living is higher
than in places like Cozumel.  Of course, I also think I get better
service there.

When I was a Unix sysadmin, the answer to a bunch of questions was
always "It depends".  The proper answer to any "why did you do that"
question is "because it seemed like a good idea at the time".