Ship tickets are tickets for the Lady of the Waves. It can be exchanged to Captain Shanks for a boat ride to Port Khazard or Port Sarim.
They can be purchased in Shilo Village from Seravel above the fishing shop north of the entrance. They can also be purchased from Captain Shanks who sells it for a varying price (usually between 26-45 coins).
Ship tickets can sometimes be found inside barrels.