I generally tip $5/tank unless there was exceptional service and/orspotting or if I needed extra help for some reason.
I do not apply this same practice to day trips (usually 3 tanks). Daytrips, I tip $25 as the base and more if the service was extra special. That's a long hard day for these guys. Think of it, we get up to the dock around 6 to catch the boat; they've already been at it getting everything ready. Then they have to look after us all the day including lunch, drinks on the way back, etc. and then clean up after we're napping in our room before dinner. I think that's worth more than $5/tank.
Hey, and if it's *not* worth more than $5/tank, then I sure wish somebody would tell me so.