I base the tip on services rendered. I’ll give that kid who humps everybody’s gear from truck to boat and back again $20 every time (and all my buddy’s do too), but the deckhand who basically sits on the boat, snags the mooring line, sits on the boat, snags the dock line - he gets nada because he’s just along for the ride (and maybe logging hours at sea toward his 6-pack license). For the average crew member who provides some assistance / value to the passengers, $5 or $10 depending on how many other divers are along.