In a medium bowl mix the marinade ingredients. Set aside ½ cup of the marinade to use as a dressing for the salad.
Place the shrimp in a large, resealable plastic bag and pour in the marinade. Press the air out of the bag and seal tightly. Turn the bag several times to distribute the marinade, place the bag flat on a plate, and refrigerate for 1 hour.
Prepare the grill for direct cooking over high heat (450° to 550°F) and preheat the grill pan for about 10 minutes.
Drain the shrimp in a fine mesh sieve. Spread the shrimp in a single layer on the grill pan and cook over direct high heat, with the lid closed as much as possible, until the shrimp are slightly firm on the surface and completely opaque in the center, 2 to 4 minutes, shaking the pan once or twice and turning the shrimp over for even cooking. Remove the pan from the grill and put the shrimp in a large bowl to stop the cooking.
Add the watercress to the large bowl. Spoon the reserved dressing over the watercress and shrimp (you may not need all of it). Toss to coat the ingredients evenly. Serve right away.