In a large bowl, combine the cubed watermelon, sliced strawberries, blueberries, and raspberries. Gently toss the berries together to mix them evenly.
In a small bowl, whisk together the lime juice, honey (or agave syrup), olive oil, chopped mint, and a pinch of salt until well combined.
Drizzle the dressing over the fruit salad and gently toss to coat all the fruit with the dressing.
Let the salad sit for about 10 minutes to allow the flavors to meld.
Serve immediately or refrigerate for up to 1 hour before serving for a chilled option.