Center oven rack and preheat oven to 425 degrees. Place chicken in a sauté pan, add water, and cover. Simmer for 20 minutes and remove from pan; let cool slightly. When chicken is cool enough to handle, shred using two forks, one in each hand. While the chicken is simmering, prepare remaining ingredients: Wash potatoes and slit each one lengthwise about 1-inch deep. Place potatoes in a microwave-safe dish and microwave on high, uncovered, for about 10 minutes. Let cool slightly. Slice each potato in half length-wise, scoop out potato, leaving a 1/4-inch shell. In a bowl, mash potato together with hot pepper sauce, sour cream, margarine, and milk. Fold in blue cheese and shredded chicken. Spoon mixture into potato skins slightly denting center (to hold fresh toppings after baking). Place filled potato skins on a 9-x13-inch baking sheet and bake about 15–20 minutes until tops are golden brown. Dice tomatoes and green onions. Cut celery stalks into four-inch sticks. To serve, top potatoes with onions and tomatoes. Serve celery sticks on the side for scooping potato contents.