Preheat the fryer. In a mixing bowl, whisk the egg, flour, cornstarch, soda water and salt. Blend until smooth. Let the batter sit for 10 minutes, to rest. Season the oysters with salt and pepper. Dredge the oysters in the flour, tapping off any excess flour. Dip the oysters in the tempura batter, letting the excess drip off. Then place the dipped oysters in the hot oil, fry until slightly golden, about 2 minutes. Remove the oysters from the oil and drain on a paper-lined plate. Season the oysters with salt and pepper.