Well, if they haven't done anything for you, don't tip them. Kind of like the annoying tip jar at the coffee shop. Why should we tip for them ringing up the coffee?
Unfortunately, tipping if way out of hand in the US, but I can never imagine one of my patients tipping me for starting their IV or bringing them their pain medicine in a timely fashion. Sheesh.
Again this is a service industry..medical care is not ..although my friends wife is a big shot doctor at sloan K hospital here in NY and some patients send flowers as a thankyou at times..just being human and showing appreciation..do not compare being a dm or instr to professions that pay well ..if the true cost of learning to dive was passed to the consumer there would be very very few people getting into the activity..and that alone will increase the cost as volume is not there.. Good Instr's will have to be making minimum of 40-50 grand a year with health care benefits /401 k plan etc...the people who have these perks are the ones who are not usually tipped,they are not in a service industry..a good waitress can make more than an instructor down in Key West,alot of instructors there after their hours working the boat/shop/teaching do in fact also work as waiters /waitresses /bar tenders at night to make ends meet. Don't see DR's /lawyers/etc having to do that..