HOW cold?

The temperature of almost all of the deep ocean is only a few degrees above freezing, 39F (4C)

HOW dark?

In the clearest water at midday, sunlight dims by one-tenth about every 250 feet (75 m). Humans can just barely see light below 1600 feet (500 m), at about one-millionth of the intensity at the surface.

HOW much pressure?

Most of the deep ocean is under pressures of 3000 to 9000 pounds per square inch (or about the equivalent of 100 to 300 times the air pressure in automobile tires).

35,802 ft (10,912 m)  At the bottom of the Marianas Trench (the deepest point on earth) the pressure is over 8 tons per square inch, or the equivalent of an average-sized woman holding up 48 jumbo jets!!!