Put the clams and 1/2 cup water in a large skillet over medium-high heat; cover and cook for 4 minutes or until the shells open. Remove the clams from the pan, reserving the cooking liquid. Cool clams. Remove the meat from the shells, and set aside.
Bring 8 cups water to a simmer in a large saucepan (do not boil). Keep warm over low heat. Put 2 tablespoons olive oil, parsley, and garlic in a large saucepan; cook over medium-high heat until garlic sizzles. Add the rice, and stir until coated; cook for 5 minutes, stirring constantly. Stir in reserved clam liquid; cook until the liquid is absorbed, stirring constantly. Add water, 1 cup at a time, stirring constantly until each portion of water is absorbed before adding the next. Continue until rice is tender. Season with salt and pepper. Add clams; cook 3 minutes or until thoroughly heated. Stir in 1 tablespoon olive oil, and sprinkle with parsley, if desired. Serve immediately.
Note: To substitute canned clams for the fresh, use 3 (6-ounce) cans clams, undrained, and use 7 cups water.