Preheat oven to 425°F. Line 2 baking sheets with parchment paper. Trim both ends of each zucchini so that it’s about 7 inches long. Cut each zucchini lengthwise into 4 equal slices (about 1/4 inch thick). Arrange zucchini on a work surface; sprinkle evenly with salt. Combine cornstarch, garlic powder, and pepper in a shallow dish. Combine eggs and water in another shallow dish; whisk until well combined. Place panko in a third shallow dish. Working with 1 zucchini slice at a time, dredge each in cornstarch mixture, shaking off excess. Dip in egg mixture, turning to coat. Dredge in panko, pressing gently to adhere. Place on prepared baking sheet. Repeat process with remaining zucchini. Coat top of breaded zucchini with cooking spray. Bake at 425°F for 15 minutes. Carefully turn zucchini slices over, and rotate pans. Bake an additional 15 minutes (for a total of 30 minutes) or until crispy on the outside and tender when pierced with a knife. Coat an 11 x 7–inch baking dish with cooking spray. Arrange half of zucchini in pan, overlapping slightly if necessary. Spread 1 cup marinara down the middle of zucchini, leaving about 1 inch of either end uncovered. Sprinkle about 1/2 cup mozzarella and half of Parmesan over sauce. Arrange remaining zucchini on top, overlapping slightly if necessary. Spread remaining cup marinara down the middle of zucchini, leaving about 1 inch of either end uncovered. Sprinkle remaining 1 cup mozzarella and remaining half of Parmesan over sauce. Bake, uncovered, at 425°F until cheese is bubbly and lightly browned, 20 to 25 minutes.