In a large skillet, cook and chop the ground chicken over medium heat until no longer pink. Season to taste with salt, pepper, and garlic powder.
Mix about 2 cups of tomato sauce with the chicken. Turn the heat to low and simmer until the sauce is absorbed and the mixture is not too runny; adjust sauce quantity if using a thin sauce.
Preheat the oven to 400 degrees F. Grease a 9x13 baking dish.
While the chicken and sauce simmer, de-stem and cut the zucchinis in half lengthwise. Use a spoon or melon baller to carve out the insides.
Mix 2 cups of shredded mozzarella and 1/2 cup of parmesan into the chicken and sauce mixture. Stir until the cheese is melted.
Spoon the mixture onto the zucchini boats. Sprinkle the remaining mozzarella and parmesan over top.
Bake uncovered on the center rack for 25 minutes. Then, switch the oven to broil for a few minutes to brown the cheese.