A number of trade publications still provide general arrangement drawings of cruise ships. The only ship that is not available is the QM2. Tell us which ships you are especially interested in nzmatt and I will post details of the publications you should buy.
The trade press publish shipbuilders general arrangement drawings for the first of class. I think your best bet is a Swedish organisation, ShipPax Information, contact them at www.shippax.se and explain your requirements. They are a friendly and helpful crew. They produce an annual Designs book in A4 page format.
From a model making viewpoint, everything depends upon the standard of the GA supplied by the shipbuilder. Designs 04 for example contains excelent profile and decks drawings of Costa Fortuna and Diamond Princess, but less useful drawings of Crystal Serenity, Caribbean Princess and QM2. The cost is US $97, but earlier years are less expensive.