To tip or not to tip, that is the question on a dive charter. Years
ago tipping was unusual; now it's a common practice. Divemasters are
not highly paid, so if you feel you received good service, a tip is customary,
and is greatly appreciated.
Moreover, if you've required special help - - having the divemaster
  gear you on the transom and haul your heavy gear out of the water or
  if the divemaster does a quick repair to your gear on the boat (usually
  for free) - - you should consider a more generous tip.  
Lastly, divemasters are supposed to be guides, not instructors. So if
  you needed a mini-refresher course in how to get down, manage your
  buoyancy, clear your ears or other basic scuba skills, you might want
  to thank the divemaster with a gratuity. And if the divemaster rescued
  you from yourself - - arrested an uncontrolled ascent or descent or
  swam you down in a current and got you back to the boat - - proper
  humility, profuse thanks and a sizeable tip will go a long way toward
  squaring your scuba karma.