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 rocket, 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.