Man overboard

is an exclamation given aboard a vessel to indicate that a member of the crew or a passenger has fallen off of the ship into the water and is in need of immediate rescue.

This is especially so if falling from a large vessel that is slow to maneuver, or from a short-handed (i.e. undermanned, lacking sufficient crew) smaller boat.

If the individual who falls overboard is wearing a personal flotation device (life jacket), the chances of their survival are significantly improved.

If the equipment exists, man overboard alerts are to be triggered on whatever electronic gear is available including GPS receivers and DSC radio transmitters.

Immediately after the accident, the boat is put onto a beam reach away from the casualty, with a crewmember maintaining constant visual contact.

It is important that the casualty is approached on a close reach, so that wind can be spilled from the sails in order to slow down and stop on station.

[6] An alternative maneuver, somewhere between the two above in terms of complexity and flexibility, is to put the boat onto a deep beam reach immediately after the accident, and sail a few boatlengths away downwind and to one side.

At this point, the boat is rounded up and tacked so that, as in the reach-turn-reach, the casualty is approached on a close reach maintaining the ability to steer, slow down and stop as required by the conditions.

The engine(s) must be out of gear before the casualty is approached, and may be switched off entirely during the actual recovery to ease communication, reduce fumes and allow people to concentrate on the task in hand.

All crew members involved in recovery should be harnessed on if the sea conditions that led to the first fall could lead to further people ending up in the water.

The condition of a person in the early stages of hypothermia can be made considerably worse by hoisting them vertically so that what circulation they have drains from their head into their feet.

Various pieces of equipment are on the market to help short-handed or weakened crews deal with this problem, but really nothing beats the combined efforts of several strong hands gripping various items of clothing on the arms, body, and legs, and hauling in unison.

In a rough sea, the waves that caused the fall can sometimes help by lifting the floating person up within easy reach of the sidedeck as the boat rolls.

Such a tackle may be fashioned from a mainsheet, a boom vang (kicking strap), or may be purpose-made for the job and stored in case needed.

Some manufacturers' hardware integrates with other systems on the boat; for example, it may activate a throttle kill switch or control the autopilot to return to the point of the downed member.

Signal flag Oscar indicates "man overboard".
Rescue (MOB on a boat)
A video on preventing, and recovering from, man overboard
Dummies are used for man-overboard drills.