Process the cucumber into cucumber spaghetti with the spiral cutter and place in a large bowl. Then peel the watermelon and cut into bite-sized cubes and add to the cucumber spaghetti. Crumble the feta cheese over the cucumber and watermelon.
Wash the mint, dry it, shake it and cut into fine strips. Add the lime juice and mint to the remaining ingredients and mix everything well. If you want, you can add salt and pepper to taste.