Prepare outdoor grill with medium-hot coals, or heat gas grill to medium hot.
In small bowl, whisk together vinegar, honey, mustard, tarragon, 1/4 teaspoon salt and the pepper. Gradually whisk in the 1/4 cup oil until smooth and thick.
Brush ears of corn and cut sides of tomatoes with the remaining 1 tablespoon olive oil.
Grill tomatoes, cut side down, and scallions, turning scallions occasionally, until grill marks appear and the tomatoes and scallions are lightly colored, about 4 minutes. Remove to platter. Place corn on grill grid. Cover grill; grill corn, turning occasionally, until lightly golden, 10 to 12 minutes. Remove from grill; let cool to room temperature.
Cut kernels from corn cobs. Cut tomatoes into chunks. Slice scallions into 1/4-inch slices. Place corn kernels, tomatoes, scallions and watercress in large salad bowl; toss gently to combine. Whisk dressing to combine. Add to salad bowl, along with remaining 1/4 teaspoon salt; toss gently to combine. Serve at room temperature with Italian bread to sop up all the juices and dressing.