Preheat your oven to 350°F (175°C).
In a large pot, boil water and cook the jumbo pasta shells according to package instructions until al dente. Drain and let them cool slightly.
In a mixing bowl, combine the cooked shrimp, crab meat, ricotta cheese, half of the mozzarella cheese, Parmesan cheese, parsley, garlic powder, onion powder, and black pepper. Mix until well blended.
Stuff each jumbo pasta shell with the seafood mixture using a spoon.
In a 9x13 inch baking dish, spread 1 cup of marinara sauce evenly over the bottom.
Place the stuffed shells on top of the marinara sauce, then pour the remaining marinara sauce over the shells.
Sprinkle the remaining mozzarella cheese on top of the shells.
Cover the baking dish with foil and bake for 20 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Allow to cool for a few minutes before serving. Garnish with extra parsley if desired.