In a large serving bowl, or platter, layer or arrange the watermelon, blueberries, feta, cucumber, and arugula.
Place the dressing ingredients into a jar or other container with a lid and shake vigorously to combine.
Pour the dressing over the salad and toss together, or serve on the side.
Notes
Add finely diced red onion or chopped fresh basil to the vinaigrette for more depth of flavor.
If preferred, substitute red wine vinegar for the white vinegar in the dressing.
If desired, skip the vinaigrette and dress with balsamic vinegar or bring balsamic and honey to a boil together, then reduce heat to a simmer until thickened. Cool completely before drizzling over the salad, then serve.