For the cheesy grits: Set a medium pot over medium heat. Add the cream and 2 cups water and bring to a boil.
Slowly add the grits, stirring to avoid lumps. Reduce the heat to low and cook, stirring frequently, until the grits thicken and look creamy, about 1 hour. Add half of the mild cheddar, half of the sharp cheddar and all of the Parmesan, and stir until everything has melted to a creamy-smooth consistency. Season with salt and pepper. Keep warm until ready to serve.
For the fried fish: Cut the fish into 8-ounce portions. Season with salt and pepper. Spread the flour out on a plate and dredge the fish in the flour, making sure all sides are coated.
Fill a deep frying pan halfway with canola oil and heat over medium-high heat. When the oil is hot, carefully slide the fish into the oil and fry for 3 to 4 minutes on each side, working in batches to avoid crowding the pan. Drain the fish on paper towels. Serve hot with spicy mayo and cheesy grits.