Brown prosciutto in a small nonstick skillet over medium heat 5 minutes. Remove prosciutto, and set aside.
Whisk together basil and next 3 ingredients; gradually whisk in oil until blended.
Arrange watermelon cubes over watercress. Sprinkle with prosciutto and pepper, and drizzle with vinaigrette. Serve immediately. Garnish, if desired.
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