Pressure is a function of the depth only, so
, where d is the depth and is the water density.
We also know that where we are integrating over the surface we want the force on.
Since the pressure is constant for a constant depth, we can break the surface into thin horizontal strips. So let x be the horizontal direction and y the vertical. P(d) doesn't depend on x, so
(Remember we need and g in English units! Since I don't have for water in those units handy, I'll leave the rest to you.)