Right after the cut scene where the main character tells Henry he has arrived at the ship, walk to the end of the ramp leading on board. Do not step on the ship yet! From the edge of the boarding platform you should be able to see a man standing around a corner straight ahead Simply pull out your bow, take aim, and kill him. Now step on the ship and deal with what happens. When you can relax again you should have an arrow in the guy you killed, and one sticking out of the tank in the middle of the area you are in, along with another goody that guy will drop.