Preheat the oven to 425°F (220°C).
If not already roasted, halve the spaghetti squash lengthwise, remove seeds, place cut-side down on a baking sheet, and roast until tender; scoop out the strands into a large bowl (you should have about 2 1/2 cups).
Stir the marinara sauce, Italian seasoning, and garlic powder into the shredded squash until evenly coated.
Divide the sauced squash evenly between the two squash shells, then sprinkle 1/4 cup shredded mozzarella on each filled shell.
Place the squash boats on a baking sheet and bake 5–6 minutes, or until the mozzarella is melted.
While the squash bakes, heat the olive oil in a large skillet over medium-high heat. Season the shrimp generously with salt and pepper.
Add shrimp to the hot skillet and cook about 45 seconds per side, until pink and opaque; remove from heat and set aside.
Remove the squash boats from the oven and top each with three shrimp, 1 tablespoon grated Parmesan, and 1 tablespoon breadcrumbs.
Return the baking sheet to the oven and bake an additional 4 minutes to warm the shrimp and lightly brown the toppings.
Carefully transfer the squash boats to a serving platter, sprinkle with chopped parsley or basil if desired, and serve immediately.