The bottom layer of the ocean is the coldest because cold water has a higher density than warm water. The colder water sinks to the bottom of the ocean. This movement of water creates the ocean currents.
Heat from the earth is transferred from the ocean floor to the surrounding water. However, this is not enough to make an impact. Geothermal energy, coming from the earth, averages 1/10 of a watt per square meter. If the ocean quit moving and no other energy came in or out, it would take a year for geothermal energy to raise the temperature of the ocean floor by 1 degree Celsius.