Spread the cubed watermelon on a large plate or sheet pan and refrigerate for 30 minutes if your melon isn't already cold. Cold watermelon makes all the difference here.

In a small bowl, whisk together the lime juice, rum (if using), honey, olive oil, salt, and pepper until the honey fully dissolves and the mixture looks slightly creamy. Taste and adjust - it should be bright, lightly sweet, and punchy.

Stack half the mint leaves, roll them tightly like a cigar, and slice crosswise into thin ribbons (chiffonade). Leave the remaining leaves whole for garnish.

In a large bowl, gently toss the cold watermelon, cucumber, and sliced mint with about two-thirds of the dressing. Use your hands or a wide spatula to avoid breaking up the melon.

Transfer to a serving platter, scatter the feta and whole mint leaves over the top, and drizzle with the remaining dressing. Serve immediately while cold and crisp.