In a large mixing bowl, combine the cubed watermelon, sliced strawberries, blueberries, and raspberries. Gently toss the fruits together to mix them evenly.
In a small bowl, whisk together the lime juice, honey (or agave syrup), chopped mint leaves, and salt until well combined.
Drizzle the lime-mint dressing over the fruit salad and toss gently to coat all the fruits evenly.
Chill the salad in the refrigerator for at least 30 minutes before serving to allow the flavors to meld.
Serve the salad in individual bowls or on a large platter, garnished with additional mint leaves if desired.