The German language has many different greetings used by different groups of people. This old tradition dates back to the end of medieval times when the guilds would not let a craftsman become a master unless he had some experience. So after finishing the apprenticeship a craftsman would set out for his wander years. As these journeymen relied on the general public for support there were many formalized ways to ask for help (shelter, food, work, etc.).
These formulas included different greetings depending on the profession or guild of the journeyman. Thus each profession has it’s own specific greeting.
In keeping with this this tradition many non-guild groups (e. g. sportsmen) started using specific greetings.
Divers use “Gut Luft!” meaning “good air” as in may you always have good air which is something you’d probably always wanted to wish every diver.
The proper response would be “Danke” − “Thank You!” or “Dir auch!” − “to you too”.
Thus, wherever you are diving: Gut Luft!