The officer of the merchant vessel calls out for a song for setting sail, and Billy Budd starts in on an appropriate chantey (if it were 50 or so years later). Funny though, I don't see anyone hauling halyards; they just seem to be standing around swaying their bodies and having a nice time. Funny too that the song is sung from the masthead!
A nice detail is the surprise shown by the British officers of the other ship, "They appear to be singing, sir." AT one point it may have been that chanties of this sort were unfamiliar on British vessels. Also, chanties being only a feature of merchant ships, naval vessels' sailors may have been unfamiliar.