Tipping an instructor is an insult. A job well done is acknowledged by a handshake. Would you tip your doctor or dentist? Tipping is for dive staff at a resort or boat crew on a trip. Although you may tip the Captain in some parts it is not acceptable in all circumstances. These people are professionals and as such should already receive adequate monetary compensation for their services. A 'gift' to show appreciation is acceptable. A fine wine, or bottle of beverage of choice, is a good option. But don't ever refer to it as a 'tip'.
I think it depends on where in the world you are. In the dive resorts the instructors earn peanuts and I think you'd have to search a long way to find one who would find a tip insulting.
On the other hand, those who teach at home as a hobby probably would rather have thanks or a bottle of something if you must rather than a tip...