For starters it is not a watermelon. You may get more assistance in the vegetable forum. your plant is a Cucumis melo not a Citrullus lanatus. Both will be marketed under vegetables. If you have slugs, that would be my first guess.

Thanks for the feedback. I was wondering about the cucumis melo actually, thanks for the clarification! We planted both but there was a mix up in the process. How do I know if I have slugs? I have never seen them around before.