A shark activist swimmer who spent 12 days swimming around Martha’s Vineyard to mark the 50th anniversary of “Jaws” revealed he broke his cardinal rule during the grueling — and frigid — 60-mile journey.

Lewis Pugh, 55, had promised himself not to think about what sharks might be lurking below him as he tried to draw attention to the fact the animals are no monsters.

“There was one day when I was getting into the water and it was very, very early in the morning,” Pugh told The Post. “The sun hadn’t even risen. It was just twilight and I’m lowering myself into dark black water.

“If you’ve been talking about sharks all day long the day before, guess what you’re thinking about when you lower yourself down that ladder?” 5

But the 55-year-old British-South African swimmer didn’t have a

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