Klabautermann

Klabautermann is a water sprite (or nix) who assists sailors and fishermen on the Baltic Sea in their duties. He is a merry and diligent creature, with an expert understanding of most watercraft, and an unsurpassed musical talent. He also rescues sailors washed overboard. His image is of a small sailor in yellow with a tobacco pipe and woolen sailor's cap. This likeness is carved and attached to the mast as a symbol of good luck. Despite the positive attributes, there is one omen associated with his presence: no member of a living ship shall ever set eyes on him. He only ever becomes visible to the crew of a doomed ship.

This is shown to be true in One Piece, as Usopp saw the spirit fixing Going Merry back in Skypiea, and in the end, due to various things happened, Going Merry was broken beyond repair and had to receive her Viking Funeral.

(Source: One Piece Wikia)