Underwater World By Alexander Safonov

At depths greater than a few hundred meters, the sun has little effect on water temperature, because the sun's energy has been absorbed by water at the surface. In the great depths of the ocean the water temperature is very cold. In fact, 75% of the water in the world ocean (the great depths) has a temperature between 0 °C and 2 °C.
Water conducts heat around twenty five times more efficiently than air. Hypothermia, a potentially fatal condition, occurs when the human body's core temperature falls below 35 °C. Insulating the body's warmth from water is the main purpose of diving suits and exposure suits when used in water temperatures below 25 °C.
Sound is transmitted about 4.3 times faster in water (1,484 m/s in fresh water) as it is in air (343 m/s). The human brain can determine the direction of sound in air by detecting small differences in the time it takes for sound waves in air to reach each of the two ears. For these reasons divers find it difficult to determine the direction of sound underwater.