Shakespeare’s description of Cleopatra’s barge is definitely a close cribbing of North’s Plutarch, there’s no denying that.
Isn’t there? I mean, Plutarch is a historical source, considered in Shakespeare’s time as authoritative: using available historical sources for a play about historical people and events is surely not cribbing. (If it were, every movie or TV show that has Good Queen Bess giving her Tilbury Speech would be ‘cribbing’!) Shakespeare took the information given in the North translation, combined it with embellishments of his own and put them into blank verse: that’s not cribbing, in my book.
It’s the wording of the two passages that are the issue. Shakespeare copies North’s words fairly closely. It’s not just that he’s repeating the historical details—which would be fine in any context or era—he’s repeating the words, not verbatim, but certainly closer than many of the others who’ve been tripped up for plagiarism in recent days. A counterexample is Macbeth, the plot and many historical details of which are taken from Holinshed, but he doesn’t copy the words. That’s a case of using a historical source that would pass modern muster.
I agree that calling the passage from A&C “plagiarism” is anachronistic, as what Shakespeare did was perfectly ordinary and expected for his era, but were a modern playwright to do it, then it would be.
And the claims that Shakespeare’s adaptations like R&J are “plagiarism” are utter nonsense.
As for the Tilbury Speech, re-enacting a historical speech by having an actor recite the words isn’t plagiarism, as the attribution is built into the context. That’s a different beast altogether.