I've been singing this song for the past five years. I first heard it sung by Waterson, Carthy on the Rogues Gallery CD. I have Hugill's book and I've trawled the net and read the entire postings here. All leaving me none the wiser to the original meanings behind the song.
Shanty's were work songs, sung to help pass the time, and keep the rhythm, for long arduous tasks.
If you have ever been in a rugby club after a game, the songs that are sung are all explicit and are versions of some well known songs. There are as many versions of rugby songs as there are of shanty's.
Where the song comes from, I have no idea. That the hogeye is a word for a vagina, makes sense. Nigger wasn't considered offensive, to whites at least, in the 1800's. That many black seamen worked on railways and had liaisons with white girls from small villages is entirely plausible.
I think it is possible to imagine the original bawdy words. If you don't get them quite right I don't think it matters.