There are probably 100's of ways to do this, but here is how I would do it: -load the extra brushes that come with photoshop. -create a new layer -switch to paintbrush tool, use one of the blotchy brushes, white color, opacity 100%, don't draw with the brush, rather "spot click" your way to something that resembles a wake. -Clean up the edges of the wake with the eraser. -Play with blending tools, etc -Merge the layers