system
Given the following post, respond as if you're commenting on Reddit:
human
Does the descent velocity of an object change as it goes deeper under water?  

Say If I was on a boat floating over the Marianas Trench and I dropped an incompressible solid steel ball over the side.  Assume there would be no contact with objects that would affect the rate of descent.  Once it achieved its terminal velocity, would the rate of descent remain constant, would it decrease or would it increase as it went deeper?

I am of the understanding that water does not compress.  Going by that understanding I presume that a cc of water at 15,000 feet deep would ideally contain the exact same amount of H2O molecules as a cc of H2O at the surface.  So the physical resistance of the water versus the object dropped into that water should remain at a constant throughout the entire descent.  

I wonder though what effect water pressure would play on that descending object.  As my pin ball descends down to its abyssal destination, would the water pressure above it force it down faster?  Would the increased water pressure below it slow its descent down?  Or would those 2 forces combine to cancel each other out leading to a constant rate of descent?
gpt
Water is indeed compressible, and whether your device is more or less compressible than water is very important when you're going deep. A steel ball would be less compressible, so as water becomes more dense, it would experience more buoyancy. The increased water density would also increase drag (but not by much). The result is that the ball would slow down by a very small amount as you descend. Pressure acts equally from every direction (on something small enough), and has no direct effect.