You're wrong I believe.
Two rocks can have the exact same dimensions and therefore displace the same amount of volume when thrown overboard, but while in the boat the heavier rock will cause the boat to displace MORE water then the lighter one, since the heavier rock will cause the boat to be submerged deeper then the lighter rock.
This also means that you could have a large rock that's JUST a little denser then water (which will displace a large volume when dropped into the water) yet doesn't displace too much water when it's in the boat since it doesn't cause the boat to submerge too much or you could have a small rock, incredibly dense, that will not displace as much volume when dropped into the water, however when IN the boat causes the boat to displace a large amount of volume.
Depending on the various characteristics of the rock and boat the level of water could rise, fall or stay the same.