Wash, scrub, and dry the potatoes well. Pierce them all over with a fork.
Bake at 400°F for 40 to 50 minutes until fork-tender, or microwave each potato wrapped in a paper towel using the potato setting.
Cut a slit down the center of each potato without cutting all the way through. Scoop out the insides into a bowl, keeping the skins intact.
Add softened cream cheese, chives, garlic powder, salt, and black pepper to the potato flesh. Mix until creamy, then set aside and keep warm.
In a bowl, combine the shrimp and crawfish. Season with 1 tablespoon Cajun seasoning, 1 tablespoon onion powder, and 1 tablespoon liquid crab boil.
In a saucepan, melt 4 tablespoons of butter. Add minced garlic and sauté for 30 seconds.
Add the seasoned shrimp and crawfish. Cook for 4 to 5 minutes, or until the shrimp are just cooked through. Remove the seafood and set aside. Pour the liquid left in the pan into a cup and reserve.
In the same pan, melt the remaining butter. Whisk in the flour and cook for about 1 minute until no dry flour remains.
Slowly whisk in the reserved seafood liquid, then pour in the half and half. Bring to a low simmer.
Stir in the remaining 1 tablespoon Cajun seasoning and 1 tablespoon onion powder.
Once the sauce begins to thicken, add the American cheese, mozzarella, Monterey Jack, and Parmesan. Stir until melted and smooth.
Return the seafood to the sauce and stir until heated through.
Spoon the mashed potato mixture back into the reserved skins. Sprinkle mozzarella over each one.
Ladle the seafood sauce over the top of each stuffed potato.
Garnish with chives or green onions and serve warm. These Seafood Stuffed Potatoes are best enjoyed fresh.