I have a copy of the 1990 paperback edition of his
History of Underwater Exploration (New York: Dover Publications) on a shelf in my diving literature bookcase.
When I looked him up on Wikipedia, as a Brit I was surprised to see the title "Sir" written before his name, considering he was a US citizen. According to the tome above, he was made a Knight Commander in the order of Isabella the Catholic by the Spanish government in recognition of his voyage from Spain to San Salvador on board the Niña II, a replica of Columbus's ship, on which he served as co-organiser and navigator.