How thick is the hull of an aircraft carrier?

Not really very thick at all. Some of the British carriers had armored decks, but their were the only ones. US carrier decks were sheet steel welded into place and covered over with heavy planking. The ship carried extra planking so damaged areas could be repaired at sea by the crew. This thin skin meant that even regular high explosive bombs could smash through the flight deck on impact and then explode as they came into the hanger deck space below. This caused the loss of several US carriers, as explosions from bombs penetrating into the hanger deck could ignite gasoline vapors from refueling planes, or detonate ordnance being loaded onto planes, such as bombs or torpedoes. These powerful secondary explosions could be ship killers. The Japanese also lost carriers to this type of hit, all those sunk at Midway, for instance. Battleships had armored decks. The weather deck was thick enough to start the fuse of an armor piercing shell, but the next deck below the weather deck was the really thick one, against which the plunging armor piercing shells were to detonate, before they could penetrate into the ships vitals and do fatal damage.