Seasoning the tomatoes an hour or two ahead of time will enhance their flavor.
Cut three-quarters of the tomatoes in half and place in a bowl. (Set whole tomatoes aside for garnish.) Season lightly with salt and pepper, and toss gently. Let sit at room temperature for about 1 hour.
In another bowl, whisk the vinegar with some salt and pepper. Slowly whisk in oil. Stir in scallions, and let sit for about 15 minutes.
Arrange watercress on six plates and top with tomatoes. Garnish with reserved whole tomatoes. Drizzle each plate with dressing and serve.