Combine the water and salt in a heavy large saucepan. Bring the water to a boil over high heat. Gradually whisk in the polenta. Decrease the heat to medium-low. Stir constantly until polenta thickens, about 5 minutes. Add the garlic and arugula, and stir until the arugula is wilted. Stir in the butter, cream, and cheese, if using. Season the polenta, to taste, with salt and pepper.
Recipe courtesy of Giada De Laurentiis