Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper and lightly coat it with cooking spray.
In a shallow bowl, combine the whole wheat breadcrumbs, grated Parmesan cheese, dried Italian seasoning, garlic powder, salt, and black pepper.
In another shallow bowl, whisk together the egg and water until fully combined.
Pat the chicken breasts dry with paper towels. Dip each chicken breast into the egg mixture, allowing excess to drip off, then coat thoroughly with the breadcrumb mixture. Press the breadcrumbs onto the chicken to ensure an even coating.
Place the coated chicken breasts on the prepared baking sheet. Lightly spray the tops with cooking spray to help with browning.
Bake the chicken in the preheated oven for 15 minutes.
Remove the baking sheet from the oven. Spoon 1/4 cup of marinara sauce evenly over each chicken breast, then top each with a slice of mozzarella cheese.
Return the baking sheet to the oven and bake for an additional 10 minutes, or until the chicken is cooked through (internal temperature reaches 165°F/74°C) and the cheese is melted and bubbly.
Remove from the oven and let rest for 5 minutes before serving.