Heat the oven to 400 degrees F and coat a large rectangular baking dish with cooking spray.
Slice the zucchini in half lengthwise, trim the stem ends, and scoop out the centers with a spoon.
Season the hollowed zucchini with the Italian seasoning, salt, and pepper, then arrange them in the baking dish.
Warm the olive oil in a large pan over medium high heat, then add the sausage and cook for 4 to 5 minutes while breaking it into small pieces.
Add the onion and cook for 4 minutes, then stir in the garlic and cook for 30 seconds.
Season the sausage mixture with salt and pepper.
Pour in the marinara sauce, bring it to a simmer, and cook for 5 minutes.
Divide the meat mixture among the zucchini shells and top each one with mozzarella.
Bake for 25 minutes. The boats are ready when the cheese is bubbly and golden brown and a fork slides into the zucchini shell easily.
Finish with parsley and serve right away.