Preheat oven to 375°F; butter or spray a 13x9-inch baking dish.
Boil shrimp in salted water for 30 seconds to 1 minute; drain and cool.
Sauté onion, celery, and red bell pepper in 4 tablespoons butter until softened.
Add water, salt, and pepper; boil, then add rice, cover, and cook until done.
Cool rice mixture; stir in water chestnuts, half and half, mayonnaise, tomato puree, lemon juice, paprika, cayenne, nutmeg, and parsley.
Layer 1/3 of rice in dish, then half shrimp, then half crab (if using); repeat layers ending with rice.
Combine breadcrumbs with remaining butter, sprinkle over casserole.
Cover loosely with foil; bake for 30-40 minutes until heated through, removing foil last 10 minutes.
Serve hot.