> We could argue that the cargo ship would sail irrespective of that passenger, but we could equally make the same reasoning for people buying last minute seats

> We could argue that the cargo ship would sail irrespective of that passenger, but we could equally make the same reasoning for people buying last minute seats that would otherwise fly empty. Sure but the cargo ship business isn't moving passenger. If they always fill all the passengers rooms, they won't decide to bring more ships into the equation. Whether there's 0 passenger or 15, the ship will still go through that same amount of fuel. Someone else did gave some good example of energy usage that a passenger will cause, like the water he will consume will need to be desalinated, or if he get sick and that require diverting a ship.