Heat a 10-in. nonstick skillet coated with cooking spray over medium heat. Whisk eggs and water. Add half of the egg mixture to skillet (mixture should set immediately at edges)., As eggs set, push cooked edges toward the center, letting uncooked portion flow underneath. When the eggs are set, spoon half of the beans and half of the onion on one side and sprinkle with 2 tablespoons cheese; fold other side over filling. Slide omelet onto a plate. Repeat. Garnish with salsa and remaining cheese.