In a small saucepan over medium-high heat, bring the balsamic vinegar to a boil. Reduce heat to medium-low and simmer, stirring occasionally, until reduced by half and thick enough to coat the back of a spoon. Remove from heat and stir in the honey. Set aside to cool slightly.

In a large bowl, combine the watermelon, cucumber, feta cheese, red onion, lime juice, and mint. Toss gently to mix.

Sprinkle with kosher salt and drizzle with the balsamic glaze just before serving. Best served fresh the same day.