You don't. Or haven't you noticed that it was smashed against the rocks? just because the man was standing next to it means that they haven't got code to hide it yet.
Instead, you follow the path next to Elvarg, not inside the dragon's area, that leads to a wall. Open the wall to find yourself in the Karamja Volcano. Or oyu could just use the home teleport to get off the island.
After the quest, your boat will be back in Port Sarim, but Klarense will have stolen it.