How do you signal the boat on Skullduggery Island?

The telescope and broken mirror are used exactly ONCE on Skullduggery island. If you speak to the woman at the far right of the island, she will tell you her husband was lost at sea. (You do not have to speak to her, only read the parchment at the Governor's house.)

Get the broken mirror from the upper right shelf of the General Store, and look through the telescope. Until you read the parchment (with the Blue Candle), the only ship you see is a pirate. But now you will see a raft, which you can signal with the mirror. This will guide him to the Fort Ridley dock, where he will give you his ship.

(see related question)