Tipping is an old topic, but past posts have said there are also countries where tipping is not expected and crew are very surprised to receive one.
The problem is also a matter of perspective. In some countries it's considered a little extra, as rewards for excellent service. In others it's considered required, and a substantial part of the income. It doesn't help that it often involves a lot of unwritten rules; don't tip the cashier, but do tip the bagger, that kind of thing. It becomes awkward when the two mix, one way or the other. Personally, I feel tipping 5$ per tank to the diving deck assistant in an area where 75$ a month is an average salary for unskilled labour, is too much.