Remove skin from avocado and cut into bite sized pieces.
In a medium bowl, combine watermelon, cilantro, red onion, feta and avocado and gently mix together.
Whisk together vinegar, olive oil, lemon juice and salt and pepper.
Pour dressing over salad and stir to coat. Top with more feta if desired.
Chill for 20-30 minutes.
Enjoy on a hot summer day. Serve over a bed of arugula if desired or mix it right into the salad!