Begin by placing the cut-up chicken in a large bowl. In another bowl, whisk together the buttermilk and water. Pour the buttermilk mixture over the chicken, ensuring that all pieces are well-coated. Cover and refrigerate for at least 4 hours, or overnight if possible.
In a separate large bowl, mix together the all-purpose flour, salt, and black pepper to prepare the seasoned flour coating.
In your deep frying pan or cast-iron skillet, pour enough oil to submerge the chicken pieces halfway (about 2-3 inches deep). Heat the oil over medium-high heat until it reaches 350°F (175°C). Use a cooking thermometer to maintain the right temperature.
Remove the chicken from the buttermilk marinade, allowing excess liquid to drip off. Dredge each piece in the seasoned flour mixture, pressing gently to ensure a good coating. Shake off any excess flour.
Carefully place the coated chicken pieces into the hot oil, making sure not to overcrowd the pan. Fry the chicken in batches if necessary. Cook for about 12-15 minutes per side, or until golden brown and the internal temperature reaches 165°F (74°C).
Once cooked, transfer the fried chicken to a wire rack set over a baking sheet to drain excess oil. Let it rest for a few minutes before serving.