Place flour in a shallow dish. Dip cheese sticks into flour to coat. Arrange on a parchment paper-lined baking sheet. Freeze 1 hour.
In a second shallow dish combine panko, parsley, and garlic powder. In a third shallow dish whisk together egg and the water. Dip cheese sticks again into flour, then egg mixture, and press into panko mixture.
Lightly coat cheese sticks with cooking spray. Working in batches if needed, arrange cheese sticks in a single layer in air-fryer basket (do not overcrowd). Cook at 400°F for 3 minutes or until golden. Serve with warmed marinara sauce (if using). Makes 12 servings (2 cheese sticks each).