In a shallow bowl, whisk the egg, water, ½ teaspoon of the salt, and pepper.
Set aside 2 tablespoons of the breadcrumbs. Place the remaining crumbs in a large zipper-lock bag. Dip the chicken in the egg mixture, then place it in the bag and shake to coat.
Preheat the oven to 400°F.
In a skillet heated to medium-high, heat 2 tablespoons of the oil. Cook the chicken for 2 to 3 minutes on each side, or until it turns golden brown. Remove and set aside.
In the same skillet, sauté the zucchini slices in the remaining oil until they are crisp-tender; drain. Transfer to a greased 9-by-13-inch baking dish. Sprinkle the reserved breadcrumbs over the zucchini. Top with the tomato slices; sprinkle with ⅔ cup of the mozzarella, the basil, and the remaining ¼ teaspoon salt. Top with the chicken.
Cover and bake for 25 minutes. Uncover; sprinkle with the remaining mozzarella. Bake 10 minutes longer, or until the cheese is melted.