Watermelon rinds are edible. A member on Atheist Discussions said that he was making pickled watermelon rinds. He posted a picture of his canning jars containing cubed rinds in homemade brine. He didn't specify whether it was sweet or sour brine, but they look good.
"The world is my country; all of humanity are my brethren; and to do good deeds is my religion." (Thomas Paine)