El Dorado hops are what you want. They are full of watermelon flavor.
The problem with watermelon juice is that it's not very thick or particularly rich. It's mostly thin in texture and very watery. Therefore, adding a quart or so to the secondary will have some effects on the final beer. I suggest looking for a really good, natural watermelon concentrate and adding maybe a cup or two to the secondary. If you have a low alcohol beer, you'll probably want to hold the watermelon concentrate at about 160F and subsequently cool it down prior to adding for sanitation purposes. I find that freshly juiced prickly pears have a definite watermelon-raspberry flavor, especially when the juice is corrected with a bit of agave, lime juice, and water.