Preheat the oven to 425°F (220°C). Roast the whole spaghetti squash until tender if not already roasted—pierce, halve, seed, and roast cut-side down about 35–45 minutes for a medium squash; allow to cool slightly.
Scoop the spaghetti-like strands from the squash shells into a large bowl (about 2 1/2 cups). Discard or reserve the shells for filling.
Add the 1/2 cup marinara, 1/2 teaspoon Italian seasoning, and 1/2 teaspoon garlic powder to the squash strands and toss until evenly coated.
Divide the sauced squash evenly back into the two squash shells and sprinkle 1/4 cup shredded mozzarella on each (about 1/4 cup per side).
Place the squash boats on a baking sheet and bake in the oven 5–6 minutes, or until the cheese is melted.
While the squash bakes, heat 1 tablespoon olive oil in a large skillet over medium-high heat. Season the 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 boat with 3 shrimp, 1 tablespoon grated Parmesan, and 1 tablespoon breadcrumbs.
Return the pan to the oven and bake 4 more minutes, until the toppings are warmed and the breadcrumbs are lightly golden.
Carefully transfer the squash boats to a serving platter and sprinkle with chopped parsley or basil if using, then serve immediately.
