Best chemical to use is a fluridone product (whitecap, sonar) $$$. Can use a diquate (Reward), will not give you the best results and is cheaper than fluridone. I have had some success using Reward on small areas of watermeal. A client told me one time that he uses a leaf blower to blow all the watermeal into a corner before he treats it with herbicides. He says the leaf blower works great moving the watermeal around his pond.

You can't judge a fish by looking at the pond--Bo Diddley