The Boat/Rock Brainteaser ft Simone Giertz part 2/3

I think the answer is that the water level drops, I did it mathematically first in my board and got to that conclusion but I think there's an easy way to picture the answer: Imagine that instead of throwing a rock you're throwing something reaaaally small but veeeery heavy (like a 10 ton rock the size of your thumb or whatever). When you have it on your hand the boat will sink A LOT, but then when you throw it the boat will not sink very much and the rock will not add too much to the volume of the water so it will drop.

I solved the problem on my board and got to the conclusion that the level will go down as long as the density of the ball is bigger than density of water, and because it sinks it must be so. If the density where to be the same then the level will stay the same (the rock would sink just a little bit so as to compensate the boat) and if the rock has a lower density than water the level would fall (the rock would be virtually floating on top of the water like air or something).