15.

Which ocean layer contains approximately 90% of Earth's seawater?

The deep ocean contains 90% of Earth's seawater!

Explanation: The deep ocean is the bottom part of the ocean, while the surface ocean is the uppermost part. These two zones are separated by the thermocline, which is a collection of layers of water where temperatures decrease rapidly the deeper you go. Once you reach the deep ocean (at approximately 1,800 meters and more), temperatures are extremely low!