While I understand the need to cover the rising cost of fuel (it's going to affect everything soon), I object to them adding it on after I have paid the cruise in full. Even if technically legal, it definitely violates the intent of contract law.
BTW, cruise ships use a form of fuel that is almost as thick as tar. Otherwise, it would just take too much room to store it. So, their cost per gallon has to be many times what we pay for gas for cars.