Preheat oven to 350°F (175°C).
Cook pasta shells according to package directions until al dente. Drain and rinse with cold water.
In a large bowl, combine chopped shrimp, crab meat, ricotta cheese, half of the mozzarella, half of the Parmesan, garlic powder, Italian seasoning, salt, and pepper. Stir until well combined. Taste and adjust seasoning if needed.
Pour about 1 cup of Alfredo sauce into the bottom of a 9x13 inch baking dish and spread evenly.
Gently stuff each cooked pasta shell with a generous amount of the shrimp and crab filling.
Arrange stuffed shells in a single layer in the baking dish over the sauce.
Pour the remaining Alfredo sauce over the tops of the stuffed shells.
Sprinkle the remaining mozzarella cheese and Parmesan cheese evenly over the top.
Cover the dish with aluminum foil and bake for 20 minutes.
Remove foil and bake for another 5-10 minutes, or until the sauce is bubbling and the cheese is melted and golden brown.
Let rest for a few minutes before serving. Sprinkle with fresh chopped parsley for garnish. Enjoy!