Submarine
Found in The Wind Waker, a Submarine is a large vessel shaped like a barrel that Link can explore in his travels across the Great Sea, and considering something good is stored away inside each of these vessels, Bokoblins on wooden rafts can often be found guarding them. Each of the Submarines is guarded by various monsters, and once Link successfully defeats them all, a ladder leading up to the area in the back of the submarine will appear leading to a Treasure Chest. All in all, there are seven of these submarines scattered across the Great Sea, with the treasures within varying from Treasure Charts to Heart Pieces among other things. If Link has collected the Submarine Chart from the Grotto located on the Boating Course, then tracking down these submarines should be much easier.