I actually did a Watermelon Kolsch that I just bottled last week. May be a strange pairing but it tastes good and it's still a bit green. Let me tell ya, Kolsch yeast puts off a stinky sulfer smell. I was worried, but heard it was quite common, so just aerated the fermenting fridge daily. ;D
I actually juiced a small watermelon, straining it through a big strainer and then through a fine strainer in my funnel. Put this in the secondary and racked from the primary. It kicked off a second active fermentation for a day or so and I ended up with a big layer of trub in the carboy. I really wanted a Watermelon nose and flavor so I also added one dram of Watermelon Extract from LorannOils.com. I'm a big fan of their products and 1 dram (aka 1 teaspoon) of their extract is usually enough for a 5 gallon batch, depending on how strong you want the flavor.
I actually racked to a tertiary for 3 days because of the wicked yeast cake in the secondary. It worked out well. It's about 10 days in now and it's got the Watermelon nose, watermelon flavor and it's still beer. Pretty nifty if I do say so myself.
A Watermelon Wheat would be a great pairing. The amount of Watermelon and juicing vs extract or both is up to you and what you're going for. I knew I wanted everyone to say "hey, that's watermelon, but it's beer!", so I went for both. The one thing to keep in mind is that if you over do it, you can't go back. Under do it and you can add some flavor via extract at bottling.