The danger in swallowing watermelon seeds is that the seed may take root in the stomach and grow another watermelon there. At first this might not seem so bad since it's like eating an entire watermelon and not have to share it with anyone. However, when that thing starts to make its way down your small intestines, it will be like a watermelon making its way down your small intestines. Ouch. If I were you, I'd stick with grapes.