I made a watermellon beer that turned out well. My SWMBO loves the 21st Hell or High watermellon...... for one, ice cold, on a really hot day, after working outside, all day. But the watermellon gets overwhelming after one and as an American style wheat.... not the greatest style to begin with.
Base malt in mine is Marris Otter, with 15% red wheat, 5% oatmeal, a little biscuit, and 2 lbs chunked watermellon in the boil, and 3 cups fresh juice pressed and strained from local watermellon after 1 week in primary. Fermented at 68* with Fermentis S04.
It has been hugely popular with all that have tried it, even non-fruit beer and non-wheat beer drinkers. Makes for a good session beer.
Only drawback is a serious lack of head retention. I chalk that up to the pectin in the watermellon as it was a fault of the commercial beer as well.