Pour water into a medium sauté pan to a level about 1 1/2 inches deep. Bring the water to a boil over high heat. Add the chicken (it should be covered by about 1/2 inch water) and reduce the heat to medium. Cover and cook for about 8 to 10 minutes or until just cooked through. Drain and discard the cooking liquid. Transfer the chicken to a plate. When it is cool enough to handle, use two forks to shred the chicken into bite-size pieces. Set aside.

Stir together the mayonnaise, basil, garlic powder, salt, and pepper in a large bowl. Add the vegetables and cooked chicken. Mix well to coat all the ingredients.

Just before serving, stir in the lettuce.