Preheat oven to 350°F. Tear the baguette into 1-inch chunks. Toss with 2 tbsp oil, salt, and pepper. Spread in an even layer onto a parchment-lined baking sheet. Bake 10 min., until golden and toasted, stirring twice.
Meanwhile, halve the cherry tomatoes. Chop the cucumber. Finely chop the onion and cut the watermelon into smaller cubes.
In a large bowl, whisk together remaining 4 tbsp oil, vinegar, salt, and pepper. Add tomatoes, cucumber, onion, watermelon, and toasted bread cubes. Toss to combine and arrange on a large platter.
Drain the mozzarella balls, tear the prosciutto into ribbons, and slice the basil. To serve, arrange mozzarella, prosciutto, and basil on top of salad.