Open-water diving

In underwater diving, open water is unrestricted water such as a sea, lake, river, or flooded quarry. It is a contradistinction to confined water (usually a swimming pool) where initial skills training takes place. Open water also means the diver has direct vertical access to the surface of the water in contact with the Earth's atmosphere.^[1] Open water diving implies that if a problem arises, the diver can directly ascend vertically to the atmosphere to breathe air. Penetration diving—involving entering caves or wrecks, or diving under ice—is therefore not "open water diving". In some contexts the lack of a decompression obligation is considered a necessary condition for classification of a dive as an open water dive, as a decompression obligation is a procedural and safety restriction on immediate ascent to the surface, but this does not affect the classification of the venue as open water.

Swim-throughs – the recreational diving term for arches and short, clear, tunnels where the natural light can be seen at the far end, and there is enough clearance that it is theoretically possible for the diver to pass through the narrowest point without contact with the sides, bottom or ceiling, are technically an overhead environment, but this is often overlooked by divers as there is no risk of getting lost inside, and the risk of entrapment is generally low.

Divers progress from learning diving skills in confined or benign water such as a swimming pool to practicing skills in open water in which the environment is not restricted to a small, controlled locality and depth, with conditions more typical of a natural body of water which might be used by divers,^[2] and the range of hazards and associated risk is significantly expanded.^[3] In this context confined water and benign water are special cases of open water, as they comply with the more general condition of unobstructed access to the surface.

Some recreational diver certification agencies use a variation on this term in the title of their entry level certification. Open Water Diver certification implies that the diver is competent to dive in unrestricted water, with various constraints regarding the conditions, and particularly that their competence is limited to diving in open water with free access to the surface.^[4][5]