>>3887752I'm trying to say nothing about density.
The rock is irrelevant. The pressure felt by the rock will be equal to the water pressure, so it's easier just to talk about the water pressure.
It doesn't matter if we're talking about a piston or a weight or whatever. If the thing exerting the force doesn't move, then there must be an equal force pushing back on it.
If the big weight is pushing down with 1000N on a 1000m^2, the water is pushing back with 1000N of force on a 1000m^2 area. 1 Pascal.
If the small weight is pushing down with 1N of force on a 1m^2 area, the water is pushing up with 1N of force on a 1m^2 area. 1 Pascal.