Yes, i get the idea - push the ugly transition out to sea. You'd need to paint over the white squares with a cloned texture rather than single colour. That sounds worth a go.
Going down the other tack, and having spent some time recently with UK ordnance survey data, i now have vector data on tidal limits throughout the uk. If only that data could be used to clip the FSET bitmap data, ideally with a nice alpha fade of course.