Light the grill or heat the broiler. In a small saucepan, combine the oil, tarragon, and garlic. Cook over moderately low heat until the garlic just starts to brown, about 3 minutes.
Remove from the heat, let the oil cool slightly, and then discard the garlic. Thread the shrimp onto metal or wooden skewers and brush with 1 1/2 tablespoons of the tarragon oil.
In a medium saucepan, bring the broth to a boil with 3/4 teaspoon of the salt. Stir in the couscous. Cover, remove from the heat, and let sit for 5 minutes. In a large glass or stainless-steel bowl, toss the couscous with the watercress, scallions, lemon juice, tomatoes, and the remaining tarragon oil, 1/4 teaspoon salt, and the pepper.
Meanwhile, grill or broil the shrimp, turning once, until just done, about 5 minutes.
Serve the shrimp skewers on top of the couscous salad with lemon wedges on the side.
Test-Kitchen Tip: Wooden skewers are handy for grilling. Just be sure to soak them in water for at least ten minutes before using, or they could burn.
Wine Recommendation: Shrimp is often best with a full-bodied white wine, but with all the watercress in this salad you'll want some acidity as well. Try a smillon from Washington State or a sauvignon blanc from California.
