Cats eat nearly anything. They love watermelon and have the world's sweetest tooth.But they should avoid watermelon. Some cats can eat reasonable amounts of watermelon without getting sick. It depends on a cat's body. If it's sensitive, it'll vomit and have diarrhoea. Too much watermelon causes "the green goat illness" or hypochlorhydria in cats. Watermelon is not a cat food and should not be given to your cat. It's a gift.