A more thorough explanation:

The marine-rescue doctrine is a rule that requires a ship to use all reasonable means to retrieve a person who has gone overboard from the water if the person can be seen. If the person cannot be seen, the ship must search for the person as long as it is reasonably possible that the person is still alive.

For example, if a crew member falls off a ship and is visible in the water, the ship must immediately take action to retrieve the crew member. This may involve throwing a lifebuoy or deploying a rescue boat to bring the person back on board.

If the crew member is not visible, the ship must conduct a search and rescue operation for as long as it is reasonable to believe that the person is still alive. This may involve using radar, sonar, or other equipment to locate the person, or deploying search and rescue teams to search the surrounding area.

The marine-rescue doctrine is an important principle that helps to ensure the safety of those who work at sea. By requiring ships to take prompt and effective action to rescue crew members who have fallen overboard, the doctrine helps to prevent accidents and save lives.