They count as fruit. They can have them, but really really small amounts. I would also suggest actually staying away from the watermelon. The pumpkin is a much better option, but also remember it can cause looser stools.
The CDTs and the Redfoot torts absolutely enjoy watermelon as well as pumpkin....but like the watermelon much more....and a couple times a year I let them whoof down as much as they can....they appear to mentally be much happier after...they are all hydrated well that it is not a problem...