Mix beets, watermelon, and cilantro together in a bowl. Whisk balsamic vinegar, olive oil, salt, and pepper together in a small bowl; pour over beet mixture. Toss to coat. Refrigerate 30 minutes before serving.
2 cans sliced beets, 1 cup chopped watermelon, 3 tbsp fresh cilantro, 4 tbsp balsamic vinegar, 2 tbsp olive oil, kosher salt to taste, ground black pepper to taste