In a medium saucepan, combine 1 1/2 cups of water with the butter and 1/2 teaspoon salt and bring to a boil over high heat. Stir in the couscous. Cover the pan, remove from the heat and let stand until the couscous is tender, about 5 minutes.
Fluff the couscous with a fork. Stir in the watercress, cover and let stand until wilted, about 15 minutes. Stir in the scallion, garlic, lemon juice, oil and parsley; season with salt and pepper. Serve the couscous at room temperature.