Watermelon Tomato Salad

• 5 cups (3/4-inch) seeded watermelon cubes
• 1 1/2 pounds ripe tomatoes, cut into 3/4-inch cubes
• 3 teaspoons sugar
• 1/2 teaspoon salt
• 1 small red onion, quartered and thinly sliced
• 1/2 cup red wine vinegar
• 1/4 cup extra virgin olive oil
• Romaine lettuce leaves (optional)
• Cracked black pepper to taste