I believe either a Basin or a Valley.
A basin is a large or small depression in the surface of land. or the ocean. A basin can be caused by plate tectonics or erosion.
This section of the ocean from floor to surface is called the water column.
trench
The ocean's surface
Ocean trenches
diatom shells settle on the ocean floor from the surface
water column
Challenger Deep.
Depth is a measure of distance between the surface and the ocean floor.
The crust.
Nemo lives in the coral reef which is near the surface of the ocean. So the drop off is the part to the ocean floor. Remember the 3 parts of the ocean: The surface, the mid-section, and the ocean floor!
70% of the size of the Earth.