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The joke in your question is funny because the listener would be fully expecting the teller to say, "she knew a surprising amount about screws."
All these 'she was only a...' jokes are invariably sexual and refer to the father's employment, as in 'She was only an Admiral's daughter but her naval was a well-known base for seamen". (You have to change the spelling but not the sound of naval and seamen mentally to get the joke.)
Thus, changing tack to speak about fish as above is just a smokescreen which makes the listener laugh precisely because he has been duped by being denied the expected.
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