In a bowl, combine the tomatoes, cucumber, watermelon, avocado, and herbs. In a spice grinder, grind the coriander seeds to a fine powder. Add the ground coriander to the tomato mixture and toss gently.
In a small bowl, whisk together the olive oil, balsamic vinegar, and salt and pepper to taste. Pour over the tomato mixture and toss to coat evenly. Taste and adjust the seasoning before serving.