Rinse watercress and drain well. Pick tender sprigs from stems and cut into 3- to 4-inch lengths; you should have 1 1/2 to 2 cups sprigs. Coarsely chop remaining watercress and stems; you should have 1/3 to 1/2 cup.
In a blender or food processor, whirl chopped watercress, olive oil, pine nuts, lemon peel, and garlic until finely ground. Add salt and pepper to taste.
Rinse fish and pat dry. Cut into four equal pieces. Set slightly apart in an oiled 10- by 15-inch baking pan. Reserve 2 tablespoons pesto; spread remaining equally over fish.
Bake in a 425° regular or convection oven until fish is barely opaque but still moist-looking in center of thickest part (cut to test), 8 to 12 minutes.
Meanwhile, rinse and stem tomatoes; cut each in half. In a bowl, mix watercress sprigs, tomatoes, and reserved pesto. Spoon equally onto four dinner plates.
With a wide spatula, transfer fish to plates. Add salt and pepper to taste. Garnish with lemon wedges to squeeze over fish.