I'm always interested in this topic because I've had some fruit beers on my list of things to make but my past efforts always turned out poorly. Each fruit is different in its flavor and strength and I could see watermelon requiring A LOT of juice because it's subtle. The thing I always mention is the sugar in the fruit/juice kicking up a secondary fermentation and drying out the beer. The other part is what you want the fruit to do for you... do you want the beer to be sweet, tart, dry, etc? There are so many variables that I end up thinking about what Denny suggested... use an extract. There are places like Olive Nation that make high-quality extracts in many, many flavors. Every time you see a beer like a chocolate peanut butter stout or a toasted-marshmallow porter... you can assume it's made with an extract.