The carbon calculations seems questionable. The author should include all energy used on board the ship, which is entirely from fossil fuels. The energy required to desalinate seawater for drinking/washing was probably greater than a flight.

Also, an additional, unnecessary person on the ship is slightly increasing the risk of diverting the ship because of a medical emergency. Imagine how much carbon would be burned if this massive ship had to turn around to drop off a sick passenger.