Underwater diving, as a human activity, is the practice of descending below the water’s surface to interact with the environment. Immersion in water and exposure to high ambient pressure has physiological effects that limit the depths and duration possible in ambient pressurediving. Humans are not physiologically and anatomically well adapted to the environmental conditions of diving, and various equipment has been developed to extend the depth and duration of human dives, and allow different types of work to be done.

In ambient pressure diving, the diver is directly exposed to the pressure of the surrounding water. The ambient pressure diver may dive on breathhold, or use breathing apparatus for scuba diving or surface-supplied diving, and the saturation diving technique reduces the risk of decompression sickness after long-duration deep dives. Atmospheric diving suits may be used to isolate the diver from high ambient pressure, and although not always recognised as a mode of diving, crewed submersibles can extend depth range, while remotely controlled or robotic machines can reduce risk to humans.

The environment exposes the diver to a wide range of hazards, and though the risks are largely controlled by appropriate diving skills, training, types of equipment and breathing gases used depending on the mode, depth and purpose of diving, it remains a relatively dangerous activity. Diving activities are restricted to depths ranging from around 40 metres (130 ft) maximum for recreational scuba diving, to commercial saturation diving maximum around 530 metres (1,740 ft) and 610 metres (2,000 ft) wearing atmospheric suits. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable can vary.

Recreational diving (sometimes called sport diving or subaquatics) is a popular leisure activity. Technical diving is a form of recreational diving under especially challenging conditions. Professional diving (commercial diving, diving for research purposes, or for financial gain) takes a range of diving activities to the underwater work site. Public safety diving is the underwater work done by law enforcement, fire rescue, and search & rescue/recovery dive teams. Military diving includes combat diving, clearance diving and ships husbandry. Underwater sports include competitive sports using free-diving, snorkeling or scuba techniques, or a combination. The term deep sea diving refers to underwater diving, usually with surface supplied equipment, and often refers specifically to the use of standard diving dress with the traditional copper helmet. Hard hat diving is any form of diving with a helmet, including the standard copper helmet, and other forms of free-flow and lightweight demand helmets. The history of breathhold diving goes back at least to classical times, and there is evidence of prehistoric hunting and gathering of seafoods that may have involved underwater swimming; however, technical advances allowing the provision of breathing gas to a diver underwater at ambient pressure are recent, and self-contained breathing systems developed at an accelerated rate following the Second World War.