In a Dutch oven or soup kettle, bring water to a boil. Add the zucchini, carrot, onion and salt. Cook for 8-10 minutes or until vegetables are tender; drain.
In a large bowl, combine the soup, sour cream, garlic powder and pepper. Fold in vegetable mixture. Combine croutons and butter; place half in a greased 13-in. x 9-in. baking dish. Top with vegetable mixture and remaining croutons.
Cover and bake at 350° for 30 minutes. Uncover; bake 10 minutes longer or until golden brown.