Underwater diving is the practice of going underwater, either with breathing apparatus or by breath-holding. Atmospheric diving suits may be used to isolate the diver from the effects of high ambient pressure, or the saturation diving technique can be used to reduce the risk of decompression sickness after deep dives.

Diving activities are restricted to relatively shallow depths, as even armored atmospheric diving suits are unable to withstand the pressures of the deeper waters of the world. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable to the diver can vary considerably. Occasionally divers may dive in liquids other than water.

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, other forms of free-flow helmet and lightweight demand helmets.

Recreational diving is a popular activity (also called sport diving or subaquatics). Technical diving is a form of recreational diving that achieves greater depths or endurance or addresses more challenging conditions than found in normal recreational diving. Professional diving (commercial diving, diving for scientific research purposes or diving 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, and may be done by professionals or volunteers. Military diving includes combat diving, clearance diving and ship’s husbandry diving. Underwater sports is a group of competitive sports using either free-diving, snorkelling or scuba technique, or a combination of these techniques.
Underwater diving was practised in ancient cultures to reclaim sunken valuables, and to help aid military campaigns. In ancient times free diving without the aid of mechanical devices was the only possibility, with the exception of the occasional use of reeds and leather breathing bladders. The divers faced the same problems as divers today, such as decompression sickness and blacking out during a breath hold. Because of these dangers, diving in antiquity could be quite deadly.

Underwater diving for commercial, rather than recreational purposes may have begun in Ancient Greece, since both Plato and Homer mention the sponge as being used for bathing. The island of Kalymnos was a main centre of diving for sponges. By using weights of as much as 15 kilograms ┬áto speed the descent, breath-holding divers would descend to depths up to 30 metres for as much as five minutes to collect sponges. Sponges weren’t the only valuable harvest to be found on the sea floor; the harvesting of red coral was also quite popular. A variety of valuable shells or fish could be harvested in this way creating a demand for divers to harvest the treasures of the sea, which could also include the sunken riches of other seafarers.

The Mediterranean had large amounts of sea base trade. As a result, there were many shipwrecks, so divers were often hired to salvage whatever they could from the seabed. Divers would swim down to the wreck and choose the most valuable pieces to salvage. These salvage divers faced many dangers on the job, and as a result, laws, such as the Lex Rhodia, were enacted that awarded a large percentage of the salvage to the divers; in wrecks deeper than 50 feet, divers received one third of the salvage and in wrecks deeper than 90 feet they received half.