In a medium bowl, whisk together the red wine vinegar, red pepper flakes, salt, pepper, and olive oil until combined. Add the red onion and toss to combine. Let the onions marinate while you prep the salad ingredients.
In a large bowl, gently toss the watermelon, feta cheese, cucumber, basil, mint, and marinated onions (without their juices). Top with more fresh herbs and flaky sea salt and serve!
For more flavor you can add a drizzle of extra virgin olive oil or balsamic vinegar.