The freezing temperature of water drops about 1 degree for every 75 of the units of pressure called atmospheres, but the change is small compared with the difference even one atmosphere of pressure makes in raising the boiling point. (One atmosphere is equal to 14.69 pounds per square inch.)
Why deep sea water does not freeze depends less on pressure than on two other factors, salinity and heat. The ocean is salt water, which interferes with the freezing process and lowers the freezing point, and at depths of a mile or so, there is considerable heating from inside the earth.
At normal atmospheric pressure, water freezes at 32 degrees. But when water is compressed to about 20,000 atmospheres and cooled, other varieties of ice form, with different molecular arrangements and linkages among hydrogen and oxygen atoms.
The form called Ice II is 12 percent denser, and Ice III is 3 percent denser. The known types go up to Ice X, beyond the infamous “Ice Nine” in Kurt Vonnegut’s 1963 novel Cat’s Cradle.
Ice IX exists, but it does not have the properties Vonnegut gave it, like forming at room temperature and being able to spread quickly.
It was discovered after the novel came out, so his version was based on good speculation.