Surface oriented surface supplied diving (Bounce diving), where the diver starts and finishes the dive at normal atmospheric pressure.
Airline or Hookah diving.
Recreational forms like snuba.
Scuba diving - The use of self-contained underwater breathing apparatus. This category includes:
Rebreather, closed-circuit or semi-closed-circuit scuba
Free diving or breathhold diving, where the diver completes the dive on a single breath of air taken at the surface before the dive.
Snorkel allows breathing at the surface with the face submerged, and is used as an adjunct to free diving and scuba.
Atmospheric diving suits and other submersibles which isolate the diver from the ambient environment. These are not considered here.
Liquid breathing systems are extremely rare and at an early experimental stage. It is hoped that some day practical systems will allow very deep diving. This is not considered here.