Suppose we are suddenly thrust into World War III. For the sake of arguement let this be a conventional war.
Could cruise ships and ocean liners be converted into makeshift destroyers and battleships? Could 747s be turned into bombers?
I belive the QEII was coverted to a troop transport durring the Faulkland Island war. I doubt you’d want to make covert cruise ships or passenger airplanes into anything more specific than cargo haulers (people and things).
You could, but it would require major modifications and the result would only be useful against unarmed/poorly armed opponents.
The Germans used armed merchant vessels as disguised commerce raiders in both WW1 and WW2, while the British had Armed Merchant Cruisers as convoy escorts to help overcome the shortage of destroyers and other real warship escorts, but these were mostly based on ships which had reinforced decks, etc. included in the designs for exactly this possible use. (Both countries offered substantial subsidies to shipping companies to build the pre-prepared ships with the provision that they could be called up for wartime service.) Adding armament to modern vessels would be a bit easier because it would be largely missile based and wouldn’t require the massive reinforcement needed under gun mounts, but you would have new problems with the automated reload mechanisms, and associated magazine space/location requirements, used by many modern guns and missle launchers. Basically the best you could do would be the equivalent of the WW2 raider - able to take on unarmed merchant vessels, and possibly able to damage or even sink a warship given total surprise and good luck, but unlikely to survive any encounter where a warship/warplane gets off any shots.
Aircraft are even dicier, as the extemporised bomb and gun mounts used in earlier conflicts like the Spanish Civil War would be very difficult to add to a modern airliner without significant redesign and modification to avoid aerodynamic or structural problems. Again, you would end up with something that would be very vulnerable to any sort of fighter or anti-aircraft defense.
Cruise ships and ocean liners:
Troop transports and hospital ships.
747s:
Troop transports and cargo planes.
Even though silly movies lie all the time, the vast bulk of any technology (especially vehicular technology) used in wartime is for transport and logistics. The most devastating strategic shortages usually are in these areas, not in armaments.