Anecdotal evidence:
I have been on a particular dive boat up in the Thousand Islands (many times) where the boat goes out weeknights for $25 for a single dive. The captain’s nephew takes a wheelbarrow and runs back and forth from each diver’s vehicle and puts the gear right in the boat, then brings it all right back to the parking lot after. I have no problem giving that kid a tip that’s 80% of what I just paid for the boat trip.
I have been on a several different boats out of New Jersey (a couple times each) where there are 12 or more divers paying $100 or more for a 2 dive trip and there is a crew of 4 who do very little other than ride along and maybe grab your fins and toss them in the general direction of your seat after each dive. At most, I tip $5 which will have to split between the 3 crew other than the captain who is getting our $100 (tip the workers not the owner).
Usually on tropical dive trips the crew consists of a driver and a DM who are employees of some shop that owns the boat, and usually they are pretty helpful and hardworking. I usually give them each $5 or $10 tip on a 2 dive run. Maybe a little less for a single dive.