In a large bowl, combine the watermelon, cucumber, red bell pepper, red onion, and cilantro. Toss gently to mix the ingredients without breaking the watermelon cubes.
In a small bowl, whisk together the lime juice, fish sauce, honey, chili flakes, and salt until well combined.
Pour the dressing over the salad and toss gently to coat all the ingredients evenly.
Let the salad sit for about 10 minutes to allow the flavors to meld.
Just before serving, sprinkle the chopped roasted peanuts on top for added crunch.