How cold are the depths of the oceans where sunlight never reaches?

The surface temperatures of the oceans range from around 40°C in shallow tropical lagoons to -1.9°C, the freezing point for sea water, in polar regions.

Most of the water in the deeper layers is very cold. Even beneath the hottest tropical regions the water at a depth of 2 to 3km rarely rises above 4°C. Deep sea creatures have had to adapt to far lower temperatures than creatures living on land.