One of the brewers here gave me a couple of his watermelon brews that tasted pretty good.
He said he used the juice form 1/2 a large melon in a 5 gal batch...it was nice and subtle, but I probably would have used more.
IMO, honey dew juice is more potent than watermelon. I'd start off with 1 whole melon then add more to taste.
I just don't know how much juice you can get from 1 melon.