If you don't have white balsamic vinegar on hand, just use balsamic vinegar, white wine vinegar, or sherry vinegar.
Cut the bread into bite-size pieces, discarding any pieces that don't have crust. You should have about 3 cups. Combine the bread cubes, tomatoes, watermelon, mozzarella and salami in a large salad bowl.
In a small bowl whisk together the mustard, white balsamic vinegar and olive oil. Season to taste with salt and pepper. Pour the dressing over the salad and toss to combine. Top with fresh basil.