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 resealable plastic bag, combine the polenta and flour. Add fish to mixture, a few fillets at a time, and tumble gently to coat evenly.
Heat oil in deep fryer to 190 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.