Hydrostatic pressure
A fluid weighs down and exerts pressure on the walls and the bottom of the receptacle which contains it and on the surface of any object submerged in it. This pressure, called hydrostatic pressure provokes, in fluids at rest, a perpendicular force on the walls of the receptacle or the surface of the submerged object whichever way they are facing.
If the liquid were flowing, the forces would no longer be perpendicular to the surfaces.
Always remember that pressure is not a force.

NB. The point of application of the forces should be situated on the surface areas. In this animation the red force is not correctly applied for reasons to do with the design.