Prepare the brine, stirring together the salt, sugar, and pepper in a medium-size bowl, then pouring in the water. Stir to dissolve. Place the whitefish in a zipper-top plastic bag and pour the brine over the fish. Refrigerate it for at least 2 hours and up to 8 hours. (The saltiness and sweetness will become more pronounced the longer the fish marinates in the brine.)</