I did a watermelon wheat 'ben the brewman' style last year. It was great, see my replys in this thread.
I decided to go a differnet route this year because I would have to wait till about now to get a decent watermelon. Instead, I followed the suggestion in this thread here: http://www.homebrewtalk.com/f75/watermelon-ale-167129/
and my version (see thread) has been bottled for a week now. It is pretty good so far! Also, this one appears to be able to pour with a head on it, something the natural Watermelon version wouldn't. Check it out!