Mediterranean Tuna, Potato, Olive, and Feta Salad with Garlic Dressing

1 1/2 pounds small red potato es (each about 1 1/2 inches in diameter)

1/4 cup water

3 tablespoons balsamic vinegar

2 teaspoons extra-virgin olive oil

1 teaspoon Dijon mustard

3/4 pound haricots verts (thin French green bean s) or regular green beans

1 red bell pepper

1/2 English cucumber

10 Kalamata or other brine-cured black olive s

two 6-ounce cans solid white tuna in water

1/3 cup crumbled feta (about 2 ounces)