Instructions
Make your way to the upper floor of the level. Play the "Mystic Melody" in front of the ancient shrine to open a new door.
Enter the door and go through the water until you reach a room with skulls and ghosts. Follow the row of rings that go up into a long tunnel. Look behind the big spiked ball to collect the first of three emeralds.
Return to the starting point of the level and enter one of the main mine shafts. There is one directly to the left of the starting point. Travel along this until you reach a tunnel with a gravel floor and lots of wooden boxes. Travel all the way to the end of this tunnel.
Keep to the top-right corner of the long hole that is filled with spiked foes. Find the emerald at the end of the hole.
Return to the starting point and move to the corner on the right. Stand in front of the waterwheel and face the starting point.
Put on the sunglasses to reveal the hidden spring. Jump on top of the spring for it to propel you upwards and into the final emerald piece. This clears Aquatic Mine on hard mode.