This is folk speech that is found in the maritime world. They are names that are given under various circumstances when sailing around the world. A shellback is a name given to someone who crosses the Equator for the first time while aboard a ship. A golden dragon is someone who crosses the dateline. The dateline is the imaginary line that runs North-South through the Pacific Ocean, on the other side of the globe as the prime meridian. Finally, a golden shellback is someone who crosses where the international date line meets the equator for the first time. These names are given to sailors to recognize their global travels. Sometimes, this is accompanied by hazing if it is the first time the sailors have received this name, such as treading water in the ocean. The informant has not crossed any of these lines by ship but was involved in the sailing community growing up and associated with people that had done this.
The informant learned about this folk speech from his uncle when he was growing up. They remember it because they had always been interested in making these achievements and traveling the world by ship. The informant always looked up to people that had done this because he thought they were hardy and real seamen/adventurers. There are other terms used for some of these accomplishments, such as a son of Neptune. There are names for people who have done none of these, like tadpoles, as well.
It is certainly an achievement to have accomplished this, but the navy and other similar organizations are notorious for hazing and rites of passage, being secluded on a boat with just other sailors. It is interesting to think about in comparison to other rites of passage within different branches of the military. Although this is a general sea term overall, it is often used frequently in navies across the globe.