Boil water in a large pot, or dutch oven with just enough water to cover the whole fish. Put the fish gently into the boiling water and add sherry and ginger slices to the pot. Remove the pot from the burner immediately, and cover it tightly to soak for 15-20 minutes. At the same time, cook the sugar and vinegar mixture in the sauce pan over medium heat stirring it until it thickens.