You know, grazie42, it certainly sounds like things are significantly different in Sweden than they are here in Deep Dixie. You *completely* failed to understand what the thread was talking about.
The boat crew (call them dive masters or whatever) on the boats around here work exclusively for tips. They set the anchor (or tie off to the wreck) and provide other services, and it is customary to tip.
I always tip decently, even though the only "service" of theirs that I usually use is mooring. If I'm feeding the fish with vim and vigor and they're kind enough to get some water to me, all the better, and if I've pulled up an anchor (or two, as I did last time) and they help me haul them onboard, I'll gladly thank them with a larger tip. In this area of the world, it's just common courtesy.
I understand that things are *quite* varied throughout the world. (If you leave a tip in a restaurant in Japan, they're likely to chase you down to return the money you forgot and left behind.) In Deep Dixie, tips are part and parcel to acceptable behavior.