Wash and dry the arugula and place in a large bowl.
Cut watermelon into 1" cubes and set aside. Watermelon has a ton of water, so you'll toss it all together when ready to eat.
In a small bowl, add apple cider vinegar, honey, minced garlic, olive oil and salt and pepper to taste (5 turns of the grinder for me!). Whisk to fully combine.
When ready to serve, toss the arugula with about half of the vinaigrette. Drizzle more dressing to your taste, being careful not to overdress. You may use it all and you may have a little left over.
Add the watermelon and toss gently to combine.
Serve immediately with feta crumbled over each serving.