Preheat grill to medium heat. Place zucchini on the grill while you prepare the filling, about 15 minutes. Heat oil over medium-high heat. Sauté onion and bell pepper until softened, 5–7 minutes. Add garlic and ground chicken or turkey. Continue to cook, breaking up meat frequently, until cooked through, about 8 minutes.
Add Italian seasoning, salt, pepper, parmesan cheese, tomato sauce, water and diced tomatoes. Stir to combine.
Remove zucchini from grill and place in a disposable baking dish or on two sheets of aluminum foil. Stuff with tomato mixture then top with mozzarella cheese.
Return zucchini boats to the grill and grill, covered, for 5 more minutes or until cheese has melted.
Preheat oven to 400°F. Prepare filling and stuff zucchini boats. Top with cheese and bake for 25-30 minutes, or until zucchini is crisp-tender and cheese is melted.