How do you defeat defeat skullduggery island?

After chasing away Captain Crawfish, return to Fort Ridley. The Governor will give you a bone shovel and the location to the Buried Treasure on the hidden island, Skullduggery. Go there and dig it up.

The main technique is to shoot them with your cannons while you maintain a safe distance from their attacks. If they contact your ship, it will be damaged, and if they remain in contact, you will...

You will need to amass your doubloons to buy larger ships, because only the largest, the Phoenix Warbird, stands a chance against Captain Crawfish.(see related question)

Although the supply of doubloons is meager in the early trading, you get more of them and get them faster as the game progresses. You will seldom be stuck with the same ship for more than a few days...

I have defeated a sea monster, but there is more than one, so it depends which one you are defeating.