I am president of Auburn Aquatics Inc. in Alabama.  We manage lakes and ponds
all over the Southeast helping individuals, clubs, and corporations produce
exceptional fishing for bass and bream.  The best treatment for watershield
that we have found is 2,4, D pellets.  It's the only herbicide that we have
found to consistently get rid of it.  Grass carp don't particularly like it
but the 2,4, D works great.  If you need any further help please let me know.