>>11470135The way boats work is by displacing water with air space until the overall density of the displaced volume is equal to water (~1g/cm^3).
Rock has a higher density than water, therefore it will be displacing more water than its own volume in order to end up displacing an equal mass and thus float the boat.
If you remove the rock from the boat and dump it into the water, that rock will only be displacing a volume of water equal to its own volume, which will be less than before. The amount of water displaced by the structural mass of the boat will not change.
Therefore the total amount of water displaced will go down, and the level of water in the pool will decrease.
As a more clear example, imagine a boat weighing one ton carrying a one cubic meter block of matter weighing 1000 tons. The volume of water displaced is 1001 cubic meters. Now the block is tossed out of the boat and into the pool. The block sitting at the bottom of the pool displaces 1 cubic meter, the boat displaces 1 cubic meter of water in order to stay afloat, therefore the amount of water displaced in total drops to just 2 cubic meters. Since 999 cubic meters of water are now no longer displaced, if the pool was 10 by 10 meters square, the water level will drop by 9.99 meters.
