The goblins find diving suits and open the well cover in the castle to find a sunken ship.
Push the Skull
Have 
Winkle go into the door in the middle of the ship.
He will push the skull partly out of the window.
Catch the Lantern Fish
Switch to
Winkle and click the left corner of the square platform at
the top of the mast in the middle of the ship.
Then have 
Fingus click the lantern in the upper-left part
of the screen. Then have Winkle catch the lantern fish
when it swims near him.
Reveal the Chest
Have Winkle use the lamp fish on the ??? in the upper-right
corner of the screen. A chest appears.
Visit the Mermaid Cave
That's all you can do here for now, so move the cursor to the bottom-right of the screen until it turns into a door icon, and click to go to the mermaid cave, or move the mouse to the top of the screen and choose the wooden sign, then click Mermaid to go to the Mermaid Cave.