First, pour the oil into a large saucepan or deep fryer to a depth of 3 to 4 inches. Heat the oil to 350 degrees.
Gently clean the mushrooms with a damp paper towel. If the stems are very large (greater than 3/4 inch), you can trim them. If the mushrooms are small, there is no need to trim off the stem.
Combine the flour, salt, and pepper in a bowl, and stir.
In a separate bowl, combine buttermilk and water.
Dredge the mushrooms in the seasoned flour, shaking off the excess, and then dip them in the buttermilk. Dredge them in the flour again, knocking off the excess flour.
Place the mushrooms gently into the hot oil, and cook them until they are golden brown. It will take about 2 to 3 minutes for the mushrooms. Then drain them on a wire rack.
Serve with your favorite sauce.