That can't be true, but perhaps you saw something about temperatures which resembled those from a nuclear-bomb instead? (The key being that much much less mass is getting heated, and stays that way for a much shorter time.)
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One quick set of reasoning is this: Energy cannot be created or destroyed, and there's no reason to think the popping-bubble is causing seawater to undergo nuclear fusion, so the limit is whatever it takes for someone to repeatably set up the situation. (So basically the energy to dropping ballast-and-bubble to the bottom of the sea, popping the bubble, and then pulling the ballast back up.) Tedious, but hardly nuclear-bomb territory.
A second approach is to imagine the collapse as a giant column of water falling like weight into the gap. Imagine a magic-glass box 1x1x1 meter holding a vacuum, sunk 10km below the surface. That's 10,000 m^3 of water and roughly ~10,000kg of mass poised to fall 1m. Gravitational potential energy: ~98 kilojoules.
For comparison, that's the energy of ~3 liters of gasoline, although getting it to explode in a similarly-simultaneous way would be tricky. (Power = Energy / Time.) In contrast, the Hiroshima explosion was ~63 terajoules.