When jumping from extreme heights, hitting the water is similar to landing on concrete, if the surface of the water isn't broken and depending on how your body hits the water.
My guess is that his belly flop ruptured some internal organs, broke his ribs possibly, and the shock from the impact caused him to inhale while briefly submerged in the water, leading to an almost immediate drowning.
Where I grew up in New England, we would go quarry jumping in the summer at a few different places. But the ultimate place we found was in Vinylhaven, ME. An island off the coast that has a dozen different quarries on it.
