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 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 open water diving, though it does not affect classification of the environment. This meaning is implied in the certifications titled Open Water Diver and variations thereof.

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.

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, and the range of hazards and associated risk is significantly expanded. 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 diver 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.

There are a few variations of open water environments with more specific names. There are also a number of named diving environments which are usually also open water environments.

The extreme case of open water is the deep open ocean, where the bottom is at a depth which is irrelevant to the diver as there would be no chance of surviving long enough to reach it. Open oceanic water is often remarkably clear, but this is not always the case. There is no natural visual reference for depth in the open ocean, and depth monitoring and control is critically important to diver safety.