Why is an ocean not considered a depression?

The Ocean is not considered a depression because it is on a different tectonic plate.

Explanation:

Oceanic plates are lower than continental plates because they are denser. Oceanic plates are also not as thick as continental plates. Therefore, the difference in height is due to different plates and not a depression. A depression is a hollow or a dip in the lithosphere. It can be caused by removing the water from the groundwater storage or the structure of the rock being compromised and collapsing inwards. You can even find depressions in the ocean, especially along some convergent plate boundaries.