Dressing: In a small bowl, whisk together the olive oil, lemon juice, champagne vinegar, salt, and pepper until emulsified.
Combine: In a large serving bowl, add the cubed watermelon, blueberries, raspberries, blackberries, mozzarella, and sliced basil.
Dress: Pour the vinaigrette over the salad and toss gently to ensure the fruit and cheese are evenly coated without breaking the delicate raspberries.
Serve: This can be served immediately for the crispest texture, or refrigerated for up to 1 hour to let the flavors meld. Garnish with a few extra fresh basil leaves right before serving.