In a bowl, combine the flour, cornstarch, and salt. Add the beer and water and stir with a whisk until the batter is smooth. Set aside.
Preheat the oil in the deep fryer at high temperature. Preheat the oven to 38 °C (100 °F). Place a sheet of paper towel or wire rack on a baking sheet.
In a bowl, dust the oyster with the flour and shake to remove any excess. Coat with the batter. Fried the oysters, about six at a time, until golden brown, about 3 to 4 minutes. Drain on the baking sheet. Set aside in the warm oven and repeat with the remaining ingredients. Serve with a dash of malt vinegar or lemon wedges.