As the cold ocean water around him turned red, Paul de Gelder’s mind flashed back to a Shark Week show he’d once seen.

“My brain screamed: ‘Punch him in the face! Punch him in the eyes!’ But as soon as I reached my arm back, I realized that I didn’t have a hand,” de Gelder, now 39, tells The Post, recalling the shark attack in 2009 that nearly killed the Australian navy diver.

Less than 10 seconds later, the bull shark lazily swam away. De Gelder struggled to the ocean’s surface, thrashing his gushing limb toward his navy ship, knowing he only had seconds before his blood loss would prove fatal.

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He emerged with his life, but his right hand disappeared with the shark — as did his right leg. A second bite had irrevocably damaged the sciatic nerve in his hamstring. Despite an extensive search in Sydney Harbour, the shark that attacked de Gelder was never found.

“During recovery, I sometimes wished the shark had just gotten it over with and killed me. I kept asking myself, ‘How could I live with half a body?’”

But as de Gelder agonizingly relearned life skills, a military training mantra kept popping into his head: Improvise, adapt, overcome.

“I could feel sorry for myself, or I could use the experience and make the most of it,” de Gelder says. That meant heading back to the navy to serve as a trainer, but he soon found the conversations he had surrounding his prosthetics had a therapeutic effect on him.

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