In-water recompression is the emergency treatment of decompression sickness (DCS) by sending the diver back underwater to allow the gas bubbles in the tissues, which are causing the symptoms, to disappear. It is a very risky procedure that should only ever used when the time to travel to the nearest recompression chamber is too long to save the casualty's life.

Carrying out in-water recompression when there is a nearby recompression chamber or without special equipment and training is never a favoured option. The risk of the procedure comes from the fact that a diver suffering from DCS is seriously ill and may become paralysed, unconscious or stop breathing whilst under water. Any one of these events is likely to result in the diver drowning or further injury to the diver during a subsequent rescue the surface.

Typically, the treatment is carried out at a depth of at least 18 metres / 60 feet with a slow ascent to the surface lasting at least 4 hours 30 minutes. The treatment is more efficient if the casualty breathes pure oxygen. To reduce the risk of oxygen toxicity, a 5 minute air break is given every 20 minutes.

The principle behind in-water recompression treatment is the same as that behind the treatment of DCS in a recompression chamber: an increase in ambient pressure will reduce the volume of the bubbles allowing better blood transport downstream of the bubbles. If the casualty can breathe pure oxygen further improvements will occur because the increase in the proportion of oxygen in the blood may keep previously oxygen-starved tissues alive and the oxygen will remove other inert gases from the bubbles making the bubbles smaller.

There may be situations when in-water recompression is contemplated, for instance, when diving in remote locations where it would be difficult to transport diving casualties to a recompression chambers because rescue transport is infrequent, unreliable or not available. In-water recompression would only be suitable for an organised and disciplined group of divers with suitable equipment and practical training in the procedure.

The equipment needed includes:

• a means of securely holding the casualty at a measured depth, such as a harness and 20 metre line with a 20 kg lead weight at the bottom and 40 litre buoy at the top
• a means of allowing the casualty to ascend slowly, such as loops in the line to which the harness could be clipped
• full face diving masks for the casualty and for an in-water tending diver including two-way communication to the surface and an umbilical gas supply
• surface supplied breathing gases including pure oxygen and air delivered to the casualty by umbilical
• voice communications equipment to the divers
• use recompression treatment "table" such as the Royal Navy 62 or 67 tables