The rocks in the foreground were positioned to lead the eye into the painting, into the surf and towards the horizon. (Note how the middle one in particular changes between the top and bottom photos.) They were painted slightly bigger than I ultimately wanted them to be so that I could paint the sea spray and foam over them, not simply up to them.
When I'd finished the rocks, I brushed the rest off the paint off my brush onto the canvas in areas where rock might show through the water. As the brush got drier, the marks got scratchier and rougher, perfect for the glimpses of rock you see through shallow sea water where there's a lot of foam.