For the oysters and prawns, heat the vegetable oil in a deep-fat fryer to 190C/375F. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Dust the oysters and prawns with plain flour.
Sift the plain flour and cornflour together into a bowl, then stir in the salt and sparkling water and whisk until well combined (the batter will be quite lumpy).
Dip an oyster into the batter, then carefully lower it into the hot oil and fry for 2-3 minutes, or until golden-brown and crisp. Remove from the oil and set aside to drain on kitchen paper.
Repeat with the remaining oysters and prawns.
For the yuzu dressing, blend all the ingredients, except the vegetable oil, in a small food processor to a purée. Add the reserved oyster juice, then slowly blend in the oil to form a thin mayonnaise. Season, to taste, with salt, freshly ground black pepper and more yuzu juice.
To serve, pile the oysters and prawns onto a serving platter, then drizzle with the dressing and sprinkle over the coriander cress.