In a shallow bowl, beat egg, water, 1/2 tsp salt and the pepper. Set aside 2 Tbsp. bread crumbs. Place remaining crumbs in a large shallow dish. Dip the chicken into egg mixture, then place in crumbs and turn to coat.
In a large skillet, cook the chicken in 2 Tbsp oil until golden brown, 2-3 minutes on each side; remove and set aside. In the same skillet, sauté zucchini in remaining oil until crisp tender; drain. Transfer to a greased 13x9 baking dish.
Sprinkle the reserved bread crumbs over the zucchini. Top with tomato slices; sprinkle with 2/3 cup of mozzarella cheese, basil and the remaining salt. Top with chicken. Cover and bake at 400° until a thermometer reads 170° about 25 minutes. Uncover; sprinkle with remaining cheese. Bake until cheese is melted, about 10 minutes.