Cover the red onion with cold water and let sit ten minutes to reduce some of the acidity. Drain and pat dry with paper towels.

Remove and discard the rind from the watermelon. Cut the flesh into half-inch cubes, removing as many of the seeds as possible. Combine with the red onion, two tablespoons of the lime juice, the peppercorns, salt, and cayenne. Refrigerate for about one hour.

Just before serving, stir in the remaining tablespoon of lime juice.

Serve the salsa with grilled, baked, or broiled chicken or fish.