Statics: Computing Submerged Surface Pressures
When working submerged surface problems in statics, remember that all submerged surfaces have a fluid acting upon them, causing pressure. You must compute two pressures: the hydrostatic pressure resultant and the fluid self weight.

Hydrostatic pressure resultant: The hydrostatic pressure resultant acts horizontally at 0.333z (z is the depth of the fluid) from the bottom of the pressure distribution.

Fluid self weight: The fluid self weight is
acting downward at the centroid (geometric center) of the area of fluid.