I'm not so sure about the tip for four when you pay for four but only go out as two. There are half as many tanks to load, half as many tanks to unload, half as many weights to loan out, half as many divers to keep track of. Otherwise a group of six should tip for four as well.
Except that the DM, who is probably working for tips only, spends the same amount of time working for less money. They could probably do better driving for one of the ride-sharing companies or waiting tables for that time. I don't mind tipping more for an empty/nearly empty boat.