Thanks for those further impressions, Lighter.

Curious though, that Brand would have written a whole song based (sort of) on the line, back in '60, and then in '67-ish only mention "Oh, and then there's *this* line BTW...which maybe I'll let you think I heard somewhere while conveniently not mentioning I wrote a song around it..."

Your guess is better than mine, but I guess...my guess...would be that Brand was in his "write bawdy songs" mode and, when composing his "Strictly for Boaters"-themed album, the idea just came to mind to fit in the "Drunken Sailor" form. The Kingston Trio had just did a similar thing of using "Drunken Sailor" as a chorus to a new song in 1959:

Early in the Morning

But that line of guessing aside--
What does this tell us about the "in bed"? If, perhaps, Brand got the line from some oral tradition, where/when do folks think the "in bed" part may have been added in?

I'd like to see who might have been earliest to record the "in bed" version. The Irish Rovers supposedly had/have a tradition of closing their live shows with "Drunken Sailor", though I don't know how far back that goes. A YouTube clip of such a performance (w/ "in bed") looks like it could have been filmed the 70s...but then again it's cheesy Irish people in Canada, for whom later decades could continue looking like the 70s....

And FWIW, though the Web certainly isn't well archived before a certain date, a search suggests that the idea about "captain's daughter" as a whip started to float around 2001, around when this thread started, and spread quickly, perhaps like an urban legend.