When a wave breaks against a sea wall containing a crack, such as might exist within the blockwork, pressure pulses can travel through the fluid and propagate into the crack. This can cause high stresses to act on the sides and roof of the crack and may even cause the constituent blocks to move. The Pressure Impulse, P, is used to model the effect of wave impact against a wall in which there is a fluid filled crack. A two-dimensional field equation is derived for P that is applicable in plane cracks of non-uniform, narrow width. This is solved for several geometries relating to cracks between constituent blocks of sea walls, in order to compare the impulsive forces with the gravitational force on a block. It is shown that a large block can be lifted due to the impulse exerted by the fluid in a crack beneath it.