In a small bowl, whisk together milk, water, salt, and black pepper. Pour the marinade into a shallow dish and lay the catfish fillets in a single layer. Turn to coat both sides and let marinate for 10-15 minutes.

In a large resealable bag or mixing bowl, combine cornmeal, flour, and seafood seasoning. Remove the fillets from the marinade, letting excess liquid drip off. Add 1-2 fillets at a time to the bag and shake to coat evenly. Repeat for all fillets.

Pour vegetable oil into a deep fryer or heavy-bottomed pan, ensuring enough to fully submerge the fillets. Heat to 365°F (185°C), using a kitchen thermometer for accuracy.

Carefully add 2-3 fillets to the hot oil at a time, avoiding overcrowding. Fry for 3-4 minutes per side, flipping once, until golden brown and crispy. Remove with a slotted spoon and drain on paper towels.

Arrange fried catfish on a platter and garnish with lemon wedges. Serve hot with tartar sauce or your favorite dipping sauce. Pair with coleslaw, hush puppies, or cornbread for a classic Southern meal.