Although a real sub could float on top of the thermal layer with engines off, they couldn't lurk below it. Because of currents and the constantly changing density of the surrounding water, hovering a submarine with engines off was impossible for any length of time over a couple of minutes. Even that was nail-bitingly difficult. However, they could control buoyancy well enough to sink in the warm layer while floating in the denser cold layer underneath and sit there with power off for hours.
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