Preparation Whisk together vinegar, shallot, zest, 1/2 tsp salt, and 1/4 tsp pepper in a small bowl. Add oil in a slow stream, whisking until combined well. Peel persimmons and cut into 1/3-inch-thick wedges. Toss watercress with persimmons and enough vinaigrette to coat, then season with salt and pepper. Do Ahead Vinaigrette can be made 1 day ahead and chilled.
Preparation Whisk together vinegar, shallot, zest, 1/2 tsp salt, and 1/4 tsp pepper in a small bowl. Add oil in a slow stream, whisking until combined well. Peel persimmons and cut into 1/3-inch-thick wedges. Toss watercress with persimmons and enough vinaigrette to coat, then season with salt and pepper. Do Ahead Vinaigrette can be made 1 day ahead and chilled.