Slitherine

One of the British Mediterranean naval units escaped through the Suez Canal on the turn before Suez fell to the Axis. On the next Allied turn, the escaped naval unit is in the hex just north of the "to Eastern Med" transit hex, having passed south through the Canal. The Axis now controls the Canal, so the Allied naval unit cannot move through the "to Eastern Med" transit hex to go anywhere else. That means the naval unit is trapped in its hex, and it is unable to move (over open water) to either the Persian Gulf or Atlantic transit hexes.

In reality, a naval unit in that position ought to be able to move into the Gulf of Suez and beyond, no?

This has been mentioned before. You need ownership of a transport loop to move through it. In this case you can see this as the Axis have been able to move overland to the other side of the Suez before the ship got there to escape and then they're trapped.

I think ships in the Suez or Kiel canals would have been sitting ducks to enemy units.

"Sitting duck" just about nails it! Well, live and learn. Had I moved the carrier through the Canal one turn earlier it would have been OK. The lesson for the Allies is: when you're in danger of losing the Suez Canal, don't wait too long before moving your ships through.