Preheat the oven to 425 degrees . On a baking sheet, toss the potatoes with 2 tablespoons olive oil; season with salt and pepper. Roast, turning occasionally, until golden, about 25 minutes; let cool.
Meanwhile, in a bowl, combine 1 tablespoon olive oil and the garlic; season with salt and pepper. Add the chicken and toss to coat.
Heat a large, heavy skillet over medium heat. Add the bacon and cook until crisp. Drain, reserving the fat in the skillet; coarsely chop. Heat the skillet over medium heat. Add the chicken and cook, turning once, until the juices run clear, about 8 minutes total. Wipe out the skillet. Let the chicken cool, then thinly slice.
In the skillet, cook the corn over medium heat, stirring occasionally, until charred; let cool. In a large bowl, combine the vinegar, remaining 2 tablespoons olive oil, the charred corn, tomatoes, onion and basil; season with salt. Add the watercress, potatoes, sliced chicken and chopped bacon; toss.