I understand that water pressure is solely a function of the weight of the water column (plus atmospheric pressure) above a given point in the body of water. Ignoring atmospheric pressure, assume that a water column above a given point at the bottom of a tank of water is 10 pounds. Assume that, half way up the side of the tank, a rigid shelf is attached to the wall that extends a foot or so horizontally out into the water. (Assume that the tank is several feet wide, so that the shelf only extends part way across). Is the water pressure at the bottom of the tank under the shelf only 5 pounds? And as soon as you step out from under the shelf, is the pressure back to 10 pounds? Or is the calculation more complicated than that, and if so, why? Stated differently, if you swim into a cave at the bottom of the ocean, is the water pressure dramatically different than outside the cave?