In a small bowl, mix buttermilk, water, salt, and pepper. Pour mixture into a flat pan large enough to hold the fillets. Spread fish in one layer over bottom of pan, turning to coat each side, and set aside to marinate.
In a 2 gallon resealable plastic bag, combine the cornmeal, flour, and seafood seasoning. Add fish to mixture, a few fillets at a time, and tumble gently to coat evenly.
Heat oil in deep fryer to 365 degrees F (185 degrees C).
Deep fry fillets until golden brown, about 3 minutes. Avoid overcrowding so fillets have room to brown properly. Fish should be slightly crisp outside, and moist and flaky inside. Drain on paper towels.