Preheat oven to 375° F. Spray baking dish with cooking spray.
Rinse zucchini and pat dry. Slice each in half lengthwise, then scoop out some of the insides, leaving enough to keep the zucchini stable and "meaty."
Chop scooped out zucchini, dice onion, and slice tomatoes in half; cut watermelon into chunks. Set each aside separately.
Heat oil in a large skillet over medium heat.
Add sausage to hot skillet and cook for 5 minutes; stirring to crumble. Add onions and cook for 2-3 minutes more.
Stir in quinoa, water, and Italian seasoning, then bring to a boil; turn heat down to a simmer, cover, and cook for 12-15 minutes until quinoa is tender and water has been absorbed.
When quinoa is done, add tomatoes and chopped zucchini; stir to combine. Replace the lid, remove from the heat, and set aside to allow the tomatoes to soften.
Meanwhile, place zucchini "boats" onto a microwave-safe dish, sprinkle with salt and pepper, and cook in the microwave for 2-3 minutes until slightly softened.
Place zucchini into prepared baking dish and fill with sausage mixture; sprinkle with Parmesan cheese.
Bake on upper rack of oven for 1-2 minutes until cheese is melted and bubbly.
Serve with watermelon on the side. (Serving size: 2 boats for adults and maybe only 1 for a child, 1/2 cup fruit)