Originally posted by econundrum
The mass of the quarter is the same in or out of the water, and in each case, it's mass is moving the water aside the same amount. There would be no change whether you were above or below the water's level before you threw the quarter in. (I believe this to be true but I have not had physics in a while).
No. TheMaster is right when he says that the level will drop. the reason is because in the boat, the quarter displaces a volume of water that has weight equal to the coin's weight, while in the pool it displaces a volume of water only equal to its own volume. Since the quarter is more dense than water, these values are not equal and the level will drop slightly.
the other assertion about the boat and the guy being submerged is of course correct (no change in level because in each case the coin displaces only its own volume), but i think in that case the guy has bigger things to worry about.