Heat a small saucepan over medium-high heat. Coat pan with cooking spray. Add corn, zucchini, onions, and 1/8 teaspoon salt to pan; sauté 4 minutes or until vegetables are crisp-tender. Remove from heat.
Heat a 10-inch nonstick skillet over medium-high heat. Combine 1/8 teaspoon salt, water, pepper, egg whites, and egg, stirring well with a whisk. Coat pan with cooking spray. Pour egg mixture into pan; cook until edges begin to set (about 2 minutes). Gently lift the edges of omelet with a spatula, tilting pan to allow uncooked egg mixture to come in contact with pan. Spoon corn mixture onto half of omelet; sprinkle corn mixture with cheese. Loosen omelet with a spatula, and fold in half over corn mixture. Cook 2 minutes or until the cheese melts. Carefully slide omelet onto a plate.