In this problem we would have to know what shape the area of the water above the drum is in order to figure out the volume of the water and thus the weight. The pressure would then be equal to that weight on the drum times the area of the top of the drum. In this problem we are only given the height of the water above, not the length, width, or shape of the water above, thus cannot determine the volume, weight, and pressure of the water on top of the drum.