Diamond Steppe Island
Diamond Steppe Island is a diamond-shaped island in The Wind Waker. At a certain angle, it looks somewhat like a ship itself. Link must use the Hookshot to get to the top of it and enter a maze with many warp pots. The reward there is the Ghost Ship Chart, which marks the island appearances of the Ghost Ship on certain moon phases. After obtaining the Ghost Ship Chart, at night a glowing ship symbol will appear also on the Sea Chart. Coincidentally, this island is one of the seven recurring destinations of the Ghost Ship.
There is also one eight-eyed Big Octo which Link can destroy for 100 Rupees.
Fishman's hint on Diamond Steppe Island:
Oh, I've got some great information, small fry! Lucky you! If you use the treasure that's hidden on that island there, the thing that vanishes as soon as you get near it... It won't vanish anymore! You know...IT! I'm talking about IT! That thing that appears on nights when a wee bit of the right half of the moon is missing! Don't you know what I'm talking about?! IT!