Hydraulic action is a major erosional process in the formation of sea caves.  As waves break against the cliff air is trapped and compressed. This exerts pressure on the bonds between particles in the rock.

 

 These bonds are also weakened by abrasion from sediment carried by the wave.

 

A third factor is the wetting and drying of water in the rock.  This allows salt to crystallise between the particles, forcing them apart.