Ring of Light

In The Wind Waker, Rings of Light can be seen floating on the surface of the sea. Some appear constantly, while others are only visible at night. As the King of Red Lions approaches a ring, it will fade from view. Once Link is close to a ring of light, a soft sound can be heard, which will become stronger or weaker depending on how close you are to exact spot where the ring of light was. By using the Grappling Hook directly above the light ring, Link is able to reach under the waves and pull up a Treasure Chest.

Although very rare, it is possible to pull up a pot instead of treasure. A similar concept exists in Twilight Princess, where Link can "catch" twigs, old boots, and even bags of Rupees while fishing.