The 73-year-old woman was airlifted to a Miami hospital. She had injuries so severe that her leg was amputated.
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"I didn't even see him approach me. He came from below, and it was just like a truck hit me. It was just like a scene out of a horror movie," Ernst said.
Ernst, an experienced scuba diver, had just finished her dive in Grand Bahama last week, and was climbing up the boat ladder.
"The shark attacked and grabbed me by the leg," Ernst said.
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Ernst then struck the shark in an effort to get it to open its jaw, and credited her friend for his quick thinking.
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"I could have easily bled to death had it not been for my friend, who put the tourniquet on my leg," Ernst said.
Last month, 15-year-old Maggie Drozdowski was bitten by what is believed to be a shark while surfing in Stone Harbor, New Jersey. She received six stitches in her foot and leg.
On the opposite coast, a new study by Shark Lab at Cal State University - Long Beach found that great white sharks are more common off the California coast than previously thought.
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"Shark attacks in general are extremely unusual. The fact that they're seeing more white sharks out there over the last couple of decades means that conservation and management is working. The fact that those sharks are not biting people, I think is excellent, and people shouldn't be afraid of them," said Patrick Rex with CSULB.
Ernst said she will continue diving, and has even been researching fins for amputees.