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In underwater diving, open water is unrestricted water such as a sea, lake or flooded quarries. It is the opposite of confined water where diver 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. 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, 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 light can be seen at the far end, 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 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 typical of a natural body of water which might be used by divers, and the range of hazards and associated risk is significantly expanded. In this context confined water is a special case of open water, as it complies 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.