Open-water diving

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.

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.

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.

It is done by scientific divers for direct observation and sampling of pelagic organisms and particulate matter, particularly the gelatinous zoo-plankton that are fragile and transparent, making them relatively inaccessible by other methods,[6] and by recreational divers for observation and photography of a range of organisms not easily seen in inshore waters.

The techniques of blue-water diving have been developed over the years to suit the conditions and address the hazards of an environment which is functionally bottomless, and has no fixed visible positional references.

The tether serves to limit the distance a diver can move away from the hub, which is typically fastened to a substantial downline supported by a large buoy at the surface, and kept vertical by a weight.

The float at the surface allows the divers to move freely in the water column within the constraint of the tether, and drift with the current.

The surface platform, generally a small boat, may be tethered to the buoy, and if there is sufficient wind to make this a problem a parachute sea anchor can be deployed to minimise drift.

If a sea anchor is deployed to limit drift, it must be kept clear of the divers to minimise the risk of entanglement, and it should be buoyed to prevent sinking in a calm.

Blue-water diving operations are constrained by water and weather conditions, including wind, sea state, current strength, visibility, and the presence of aggressive predators.

[10] Black-water diving is often done as a photographic opportunity for recreational divers as there can be a wide range of plankton that would not often be seen by day or closer inshore.

[10] A wide range of animal life may be seen, including many species that spend the daylight hours at depths below those accessible to ambient pressure divers, and migrate vertically through the water column on a diurnal cycle.

The term is often used to refer to a swimming pool or tank where initial skills training of divers takes place, which is both confined and benign.