Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked rice, shrimp, crab meat, frozen peas, cream of mushroom soup, milk, garlic powder, onion powder, black pepper, salt, and paprika. Mix well until all ingredients are evenly coated.
Transfer the mixture to a greased 9x13 inch baking dish, spreading it evenly.
In a small bowl, combine the breadcrumbs with the melted butter. Sprinkle the breadcrumb mixture evenly over the casserole.
Top with shredded cheddar cheese.
Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly and the cheese is melted and golden brown.
Remove from the oven and let it cool for 5 minutes before serving.