After obtaining the Phantom Sword, head to the Ocean King Temple. If you head to the middle area of the temple, you can fight Phantoms, destroying them by targeting their weak points with your sword. In room B7-9, after all of the Phantoms are defeated, a chest will appear with a ship part within. You can then go up the next three floors to get 100, 200, and 300 rupees. You can sell the part you got for more rupees if you desire. You can then be done over and over again, as needed.