Cut the watermelon into 1-inch cubes, removing any seeds if necessary.
Thinly slice the English cucumber using a sharp knife or mandoline.
In a small bowl, whisk together the olive oil and fresh lime juice until emulsified.
In a large mixing bowl, gently toss the watermelon cubes and cucumber slices with about two-thirds of the dressing.
Arrange the dressed watermelon and cucumber on a serving platter or in a shallow bowl.
Crumble the feta cheese evenly over the top of the salad.
Scatter the torn mint leaves over everything.
Drizzle the remaining dressing over the salad.
Finish with a generous pinch of flaky sea salt just before serving.
