Watermelon contains a good amount of sugar, so expect to gain all of your water weight back immediately. If you really are committed to keeping it off, you want low to zero carbohydrates and high fat, preferably from an animal source. If you really think you need something light, a good bone broth would be fantastic. Otherwise if you’re really opposed to eating animals, you can fall back on olives, avocados, or macadamia nuts to start.