Well, as always, there are different theories. One theory says that men who sailed around the Cape Horn wore an earring in their left ear. The ear that is closest to the cape, on an eastbound voyage.

Slate wears an earring here, first row right, next to the man with the braid, Master and Commander

Should a man go overboard or his corpse not reach the bottom of the sea, the earring should be used to pay for a Christian burial. Other sources say that wearing an earring prevents drowning and brings luck. It is also supposed to promote foresight. Which if you hit an acupuncture point, besides a persistent headache perhaps also made this possible. ;)

Another possibility was to wear an earring instead of a wedding ring.  But what is possible by out, and perhaps also the simplest of all theories, that the sailor wore the ring or rings only to look good. Men at sea also had a certain amount of fashion awareness.

Drawing of a Sailor with an earing on the right side by James Gillray                    1772-1794 © British Museum               

As so often it can also be a mixture of everything that made a sailor wear one or more earrings. Exactly that can never be said, unless there is clear evidence or written documentation.