This was delicious. Key is that your watermelon must be ripe and flavourful. Did adjust the sugar down as we find fruit cocktails a bit sweet for us. You get hints of basil, watermelon, and cucumber. Gin add some herbal notes. Also added a few drops of Difford's Daiquiri Bitters. Tamed the sweetness and added some additiona herbal tones.
Although I doubted this cocktail was convincing. I added a barspoon (0,5cl) lime juice (could be even more) and could also imagine some herbal liqueur as addition.