Stan Hugill credits Jean L. Postollec for this song. This seems to be for the book Chansons de bord, a notebook of songs (1940) by Jean Louis Postollec & Jean La Pipe, quartermaster of La Scabreuse.
The song describes the footrope on a yard (spar across a mast) failing such that a sailor falls into the sea. The hemp ropes of early ships would rot in the harsh, salty conditions unless they were well-tarred. In later days, the footropes were wire and could be stretched with a capstan to test for faults.
Hugill explains that it was not uncommon for ropes to fail, and a falling sailor could survive if he landed in the sea or a sail, but his chances if he landed on deck were "nil". The shallop lowered here is the rescue boat, as the sailor in the song has disappeared into the waters.