Grease a 9-inch square baking dish. Bring the water to a boil in a medium saucepan. Slowly add the cornmeal, whisking constantly, and bring to a boil over moderately high heat. Reduce the heat to low and cook, stirring often with a wooden spoon, until the polenta is very thick, about 15 minutes. Stir in the cheese and butter and season with salt and pepper. Scrape the polenta into the prepared dish and smooth the top, using a rubber spatula. Press a piece of plastic wrap directly on the polenta and chill until firm, at least 2 hours.