I used this method with great success in a gose last year.
Mash up a watermelon and strain into a plastic gallon water jug. Collect 1/2 gallon of juice.
Invert frozen jug into one quart mason jar and allow to thaw to collect one quart of watermelon concentrate
Repeat thawing to collect 16 oz. of watermelon concentrate
Use concentrate at kegging/bottling to taste
I used the 16 oz in a 5 gallon batch which added a pleasant but not overpowering watermelon flavor.
Not my original idea, worked very well though.
Original forum post HERE