I concur with the $10, but depends a lot. If they help carry and load your gear, certainly. If I do it all, then minmum is $5/dive. A $20 tip is very common for good service, and I do that whether 2 or 3 tank trip; most are 2 tank, which is $10/dive. If there's a gear breakdown and they fixed it, saving a dive, or I have a lot of camera equipment and they provided extra service, then tip more.