Hi Richie,
My guess was that since stanza 8 can't be the one with Charles Stewart, which came next in the old broadside text but has already been used as stanza 6 here, stanza 8 might be something involving the captain. Searching for "captain", I got this. It's stanza 8 of something and it seems to have the right stanza form, and it fits the story. The only problem is that it belongs to another ballad about a sailor and a ghost:

8. "Oh, Captain, Captain,
Stand my defence,
For yonder comes
The spirit hence.
It'll cause you and all your
Seamen all for to weep,
When you are slumbering
In the deep."

It seems to be from the top of page 59. Would that be a possible match with what you found? I'm afraid it's the best I can do. Without access to the book itself, it's probably impossible to know. How do you know there is a stanza 8, by the way?