Can't remember the name of the song now, in which the sailor's "Spy a fair mermaid, with a comb and a glass in her hand." Preparing herself for company, because the ship is going to sink.

"Three times round, went our gallant ship,
Three times round went she.
Three times round, went our gallant ship,
Then she sank to the bottom of the sea."

Dickens didn't mean her as some kind of harbinger? I haven't read this novel, so I'm just contributing what little I know of mermaids (other than the text of "The Eddystone Light" which has an entirely different outcome).

SRS