The barrel price for crude is the deciding factor. Gasoline could be $6 a gallon but if the crude price is still around $100-$110 I do not see the cruise line supplement being imminent. Remember, gasoline and diesel pump prices have many factors involved such as refinery outages each spring for maintenance and to switch over to the more costly summer fuel blends legally required in many places.
Cruise ships normally burn 'Heavy Bunker C' fuel which is the lowest grade fuel, sludge really, that comes from a barrel of oil. They'll use diesel when in port or where required by local law, such as territorial waters, especially in Alaska and California.
All of that said, I agree that any fuel supplements that do reappear should not be retroactive to those who have already paid for their cruises. However, the contract language in most cases does give the cruise line the right to charge it to every passenger.
As for air fare...I hate flying these days. Cruise Fanatic was telling me how airfare to south Florida from Nashville has gone up something like 400% in the case of one airline, in less than a year.