You think that the pool's water level will lower? That's absolutely right!
The reason is that, when the rock is in the boat, it displaces its total
weight. If it weighs ten pounds, for example, then it's making the boat ten
When the rock is sitting at the bottom of the pool, on the other hand, it
displaces its volume. As explained in Buoyancy Basics and in the answer
to the Buoyancy Question, an object sinks when it weighs more than the
water it displaces. To follow the example given above, the ten-pound rock may
only displace three pounds of water. (A given volume of rock is typically three
to five times heavier than the same volume of water.)
At any rate, the rock displaces more water when it's in the boat than when it's
in the water, and so the pool's water level is lower when the rock is in the
By the way, this is the question that the physicists J. Robert Oppenheimer and
George Gamow answered incorrectly.