BTW, nobody has produced even a claim from any shanty collector or sailor-author that "in bed with the captain's daughter" had any secret meaning at all.

It's fascinating to see people insist that what's said in plain words makes more sense as code for something else.

(Assuming that's what I mean.)

In A Study in Scarlet, Watson "keeps a bull pup." Since no dog is ever mentioned again, readers have asserted confidently that it means something else: for example, that Watson has a bad temper, or carries a gun, or something equally unexpected.

The obvious interpretation, that he owns a dog (which Conan Doyle later forgot about) is ignored.