I see Theo picked up on my point. \:\) I agree that hydrostatic pressure is just that. You can think of it as the force exerted downward by the water in the Miss. River that keeps New Orleans from sinking in the mud. But the question assumes at least 2 types of pressure.

Hydrostatic pressure which has been well defined in the posts above. But in the context of the original question there is inferred a second type of pressure. Theo called it Shear pressure , I used dam failure pressure. I think Otto was talking about a keyway/core strengthening the dam and thus reducing the risk of a second type of pressure event i.e. reaching the breaking point pressure of the dam. Better stated as -- a core/keyway will reduce the chance of shear pressure being reached and exerted on the dam or it will reduce the pressure (chance of a leak/break) on the dam.