Many airlines offer vacation packages which may include car and hotel rentals, e.g. http://www.*********/. In these packages, customers get the discounted package airfare. The cruiselines should partner with the airlines and offer their cruises through the airlines vacations departments. Similarly, the cruiselines should have a reciprocal agreement where they can provide the package airfare to their clients directly.
After many cruises, I have yet to purchase air through the cruise lines because it is more expensive than what I can buy on my own. This would seem to be a win-win for both. They could even enter into exclusive arrangements as Disney does, e.g. RCL & American, Carnival and Delta, etc.
I know I would like to buy air & cruise at the same time from one party.
These "partner" deals almost always entail the airline giving up revenues.
Period. In an era when it would be cheaper to give you cash and ground the airplane because of fuel prices, do not look for an airline to jump at the chance to reduce their yield even further.