Open-water diving

Open-water diving is underwater diving in an open water environment, where the diver has unrestricted access by way of a direct vertical ascent to the breathable air of the atmosphere. Other environmental hazards may exist which do not affect the classification. Open water diving mode implies that if a problem arises, the diver can directly ascend vertically to the atmosphere to breathe air, so it is also understood that, with this restriction, a staged decompression obligation is incompatible with the open water diving mode, though it does not affect classification of the environment. This meaning is implied in the certifications titled Open Water Diver and variations thereof.

Open water environment

In underwater diving, open water is unrestricted water such as a sea, lake, river, or flooded quarry. It is a contradistinction to an overhead environment, where there is a physical barrier to direct vertical ascent to the surface, and to a flooded confined space where there may not be enough room to maneuver freely. In open water the diver has direct vertical access to the surface of the water in contact with the Earth's atmosphere.

Environments which by definition are not open water include overhead environments, and diving in these is called Penetration diving. This may involve entering caves or wrecks, or diving under ice or the hull of a large ship. 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.