To make the wasabi granita, stir water, sugar and lime juice in a small saucepan over medium heat until sugar dissolves. Simmer for 5 minutes. Stir in the wasabi paste then set aside to cool.
Transfer the wasabi mix to an airtight container and place in freezer for 30 minutes or until icy around the edges. Use a fork to break up the mixture. Cover and place in freezer for at least 4 hours, using a fork to break the mixture up every 1/2 hour.
Divide soy sauce, wasabi granita and ginger among the oysters to serve.
Serves 6 - entree or starter.