In a resealable plastic bag, mix the flour, salt, and pepper. Place the liver slices into the bag one at a time, seal, and toss to coat.
In a bowl, mix the water, ketchup, brown sugar, vinegar, Worcestershire sauce, and garlic powder.
Heat the oil in a skillet over medium heat. Brown the liver slices on both sides. Pour the sauce mixture over the liver. Bring to a boil, reduce heat, and simmer 20 minutes, until liver is tender.
