Preheat the oven to 425°F (220°C). Roast the whole spaghetti squash until tender, then halve and shred the flesh with a fork.
Scoop the shredded squash (about 2½ cups) into a large bowl. Add 1/2 cup marinara sauce, 1/2 teaspoon Italian seasoning, and 1/2 teaspoon garlic powder; stir to combine.
Divide the sauced squash evenly between the two squash shells. Top each half with 1/4 cup shredded mozzarella.
Place the squash boats on a baking sheet and bake in the oven 5–6 minutes, until the cheese is melted.
While the squash bakes, heat 1 tablespoon olive oil in a large skillet over medium-high heat. Season the 6 shrimp generously with salt and pepper.
Add the shrimp to the hot skillet and cook about 45 seconds per side, until pink and opaque; remove from heat and set aside.
Remove the baking sheet from the oven. Top each squash half with 3 shrimp, 1 tablespoon grated parmesan, and 1 tablespoon breadcrumbs.
Return the pan to the oven and bake 4 minutes more to warm the shrimp and lightly brown the breadcrumbs and parmesan.
Transfer the squash boats to a serving platter and sprinkle with 2 tablespoons chopped parsley or basil, if using; serve immediately.