1. Whisk rice wine vinegar, lime, sesame oil, and soy sauce in a small bowl.
2. Toss watermelon and cucumber in a larger bowl and add dressing a few tablespoons at a time until coated, tasting as you go.
3. Add sesame seeds and chopped herbs just before serving, toss until well incorporated, and taste for seasoning. Serve slightly chilled but not straight out of the fridge.