Whisk together the olive oil, vinegar, Dijon, and garlic in a small bowl and season to taste with salt and pepper.
Toss watermelon, cucumbers, feta, and cilantro in a serving bowl, drizzle with the dressing, toss again, and serve.
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I highly recommend Tuscan cantaloupes if they are available to you because they are deliciously more flavorful and sweet than the average cantaloupe.