Prep the produce. Cut the watermelon into bite-size cubes.
Slice the cucumber. Thinly slice the red onion. Chop the mint or basil.
Keep everything cold if possible; it helps the salad stay crisp.
Make the dressing. In a small bowl, whisk together olive oil, lime juice, honey, salt, and black pepper. Taste and adjust: add more lime for brightness, honey for balance, or salt for pop.
Tame the onion bite. If your onion seems strong, soak the slices in cold water for 5–10 minutes, then drain and pat dry. This keeps the flavor clean and mild.
Combine gently. In a large bowl, add watermelon, cucumber, and red onion.
Drizzle about two-thirds of the dressing over the top. Toss gently with a spatula or your hands to avoid crushing the fruit.
Add feta and herbs. Sprinkle in the feta and mint (or basil). Add the remaining dressing and toss once more, very lightly.
You want everything coated but not soggy.
Finish and serve. Taste and adjust seasoning. Add a pinch more salt if the watermelon is very sweet, and a few turns of pepper. Serve immediately, or chill for up to 30 minutes to let flavors mingle.