For the following reasons:

1. It's buried in conversation.

2. It's not identified as a sea song of any kind.

3. It's near the end of a long and, for most people, tedious and confusing avant-garde novel that has nothing to do with the sea.

4. If he'd got it from Joyce, he'd more likely have made it part of his text of "Leave Her, Johnny."

5. Probably too he'd have mentioned that an "uncamouflaged" stanza had appeared in such an unexpected place, had he known of it.

6. He'd almost certainly have said something about "Johnny Lever" as an odd variant of the title and the chorus, not to mention the form of the chorus.

That's enough evidence for me. There is no likelihood whatsoever that Hugill got the stanza from Joyce and then, for some obscure purpose, cleaned it up and plugged it into a completely different song.

Gibb, I think its appearance in three different songs, combined with its unusual earthy wit, suggests that the couplet was as well known as any. Until recently, it was clean it up or omit it, so three appearances is really, er, a "lot."