Why wartime canal transits are dangerous for carriers

Are there European waterways that an aircraft carrier should never transit?

Not absolutely, but canals such as Kiel or Suez can become unacceptable carrier chokepoints in wartime depending on the enemy.

The safety of a carrier route depends on war, peace, and the opponent, so no waterway merits an absolute prohibition. A peacetime canal transit may be practical, while the Kiel Canal would present an obvious wartime danger. Moving an American carrier through Suez could also become reckless when an enemy can prepare an attack around a predictable chokepoint. Route planning must combine geography with the current threat instead of turning a peacetime rule of thumb into a universal rule.