As Melville pointed out, if a sperm whale can spend more than an hour swimming at depths of up to 6,000 feet, and a sperm whale is composed mostly of gas (air-filled lungs and blubber), then Boyle's law would dictate that, as Spruch puts it, a whale of regulation size at the surface should, at the pressures a thousand fathoms down, decrease its volume to the size of a bathtub!
A sperm whale can go to 6,000 feet easily.