How to Make It
In a medium saucepan, heat 1 tablespoon of the olive oil. Add the shrimp shells, season with salt and pepper and cook over moderately high heat, stirring occasionally, until the shells start to brown, about 3 minutes. Add 2 cups of water and bring to a boil. Cover and simmer over low heat for 15 minutes.
Strain the shrimp stock and return it to the saucepan. Boil over high heat until reduced to 2 tablespoons, about 10 minutes. Pour the reduction into a small bowl. Stir in the vinegar and 3 tablespoons of the olive oil and season the dressing with salt and pepper.
In a large skillet, heat the remaining 1 tablespoon of olive oil. Add the shrimp and season with salt and pepper. Cook over moderate heat, tossing, until just cooked, about 3 minutes.
In a bowl, toss the watercress with the dressing. Add the shrimp and toss. Mound the salad on plates and serve.