It is likely impossible to outdo Stan Hugill's description here:
If one listens closely to the air of this song it will be noted instantly that the first half is of the tune of the English shanty Fare Thee Well and the second half that of Blow the Man Down. Originally sung, according to H. Jacques, by les baleiniers des mers du Sud (whalers of the South Seas), as its whaling content implies, in later years it became the property of the French Cape-Horners engaged in the Chilean saltpeter trade. It would be the shanty used aboard the ships of the famous Gallic sailing-ship companies, when they were heaving up their anchors for the last time, homeward bound from the Flaming Coast of Chile, scupper deep with saltpeter, nitrate, and guano. The verses here have been bowdlerized a little by Captain Hayet. The third refrain would be given a "hitch" on the oula by an experienced crowd of sailormen.