In a medium bowl, combine egg whites, eggs, water and salt. Beat with a wire whisk or rotary beater until well mixed. Stir in Canadian bacon and, if desired, green onion. Lightly coat an unheated, large nonstick skillet with nonstick cooking spray. Preheat over medium heat. Add egg mixture to skillet. Cook, without stirring, until mixture begins to set on the bottom and around edge. Using a spatula or a large spoon, lift and fold the partially cooked eggs so the uncooked portion flows underneath. Continue cooking for about 2 minutes or until egg mixture is cooked through but is still glossy and moist. Remove from heat immediately. Open pita halves to form a pocket. Fill pita halves with egg mixture. If desired, sprinkle with cheese.
