Born in Williamstown, Mass., William Livingston Alden (1837-1908) was just old enough to have learned his shanties in the 1850s, but neither _Who's Who_ nor his obituary in the N.Y. Times suggests that he took a sea voyage before 1885, when he was appointed U.S. Consul- General in Rome by Pres. Cleveland. He held the post till 1889.

Wiki warning: Despite the authoritative sources, Wikipedia's brief bio claims he held the office of Consul-General for the rest of his life - another indication of Wikipedia's unreliability. (If they could get that wrong....) What he did do was to remain in Europe, living in Paris and London while writing professionally.

Alden practiced law during the Civil War, then became a journalist and editorial writer. He wrote humorous editorials for the Times for
a number of years, and was well known for his books for young people and a biography of Columbus. He helped introduce sport canoeing to the United States in the early 1870s.

Unfortunately, we know nothing about when Alden learned his shanties. His Harper's article suggests that he'd heard them sung often - quite possible for an interested journalist living in New York City.

On the other hand, he didn't move to NYC, apparently, till the 1860s.
Until then, all his residences seem to have been landlocked.

It seems as though "thirty years ago" was a literary device and that none of Alden's shanties can be dated that far back on the basis of his 1882 article.

You can't win 'em all. Obviously.