Feivel that it exactly! Where I read it it assumed the rock is denser than water, and so it displaces more water inside the boat (the amount of water equal to it in weight) than it does outside (the amount of water equal to it in volume)
If the rock is as dense as water, or less dense, it would displace the exact same amount of water inside and outside – the amount which is equal to it in weight.
Meno why is this incorrect?