“Man overboard !” is the traditional cry for when a sailor or passenger on an ocean-going vessel falls over the side into the sea. The purpose of the cry was to alert the captain and helmsman of the trouble so they could decide what to do. The procedure for this event differs from ship to ship. Nowadays it is always the policy to attempt a rescue, usually by means of a life buoy.
In the past (sailing ships) whether the fallen man would be perused or not depended mostly on the weather. If it was possible to manoever the ship into the correct position immeditatly upon hearing the cry then an attempt would usually be made. In these circumstances it was expected that the entire crew remain silent to enforce discipline. A boat would be carefully lowered into the water, (hopefully without pitching the men out or swamping it), crewed by the men with the sharpest sight, they would then begin the search for the man. Despite some film references they did not ever just dive in with a rope.