Follow them.

You quietly sneak onto the deck. Funnily enough, every crew member seems to be looking the other way.

You dive into the water and swim to shore, and is barely beaten by the rowboat.

Jack Sparrow is on the boat along with some of his pirate mates. He does not seem suprised to see you, however.

His pirate mates dig up the treasure while you and Jack just stand there, looking each other but not exchanging any words.

Finally, the mates dig up the treaure chest. However, Sparrow does not have the key!