Get yourself one of those edging tools that looks like a garden hoe
without the bend. Basically, its big putty knife looking thing with a
long handle. It allows a lot of momentum to scrape them. You will
also find it useful for removing the barnacles that form around your
waterline and you can scrape them while the boat is in the water.
Often, you can even use it to reach your prop removing fouling while
you lie on the dock.