maverick: forgive my hazy memories, but I thought the ongoing joke was quite the other way around: they are all noblemen living a life as pirates

The "concealed noblemen" gag isn't ongoing. Only at the very end of the operetta does Ruth pull out that claim (that the pirates are actually noblemen), as a deus ex machina to save the pirates from being punished for their life of crime. Until the very end of the play you have no notion that the pirates may supposedly be, as Ruth claims, "all noblemen who have gone wrong".

The Pirate King's song, near the beginning, sets the tone that the pirates are not aristocracy: "Many a king on a first-class throne, / If he wants to call his crown his own, / Must manage somehow to get though / More dirty work than ever I do."

EDIT: Mav, forgive me - my quotes were right but my conclusion was wrong. Although the "fact" that the pirates are noblemen isn't revealed until the end, it is foreshadowed from the very start. (The play opens with a song whose first line is, "Pour, oh pour, the pirate sherry" - and what kind of a pirate would drink sherry, of all things?)