The OP speculates as to whether RLS was writing of 'a real shanty':- just to remind, a shanty is a work song, while the song as described would have been a forebitter, or off-duty song, or focsle song, not a shanty. Stevenson does describe shanty singing in an earlier chapter (ch 10) of Treasure Island, when the crew at the capstan ask Silver for a song to assist in their heaving and he sings the "15 Men On The Dead Man's Chest" theme song of the novel, with its chorus "Yo Ho & A Bottle Of Rum", to which the men heave - a song which does appear to be Stevenson's own invention. This would have been somewhat early for shanty singing, in fact, according to Hugill, as Treasure Island is set in about the 1750s or 60s - Dr Livesey had served at the battle of Fontenoy, 1745, some years befor the novel begins - whereas shanty singing was a feature of the merchant service [never of the Royal Navy, USN, &c] from second decade of 19C & onward.