Actually, this is quite realistic and is a convention used in all WWII strategy games that I have seen (board and computer).
The Strait is 8 miles at its narrowest, and therefore would be suicidal for surface ships to traverse. As far as I know, no Axis surface ships tried to pass either way. In addition to shore guns, mines, etc., the British also had planes based there as well.
Even submarines had a hard time entering, the Strait being guarded by destroyers, nets, and planes. See
http://uboat.net/ops/mediterranean.htm for more details. The submarines that did enter did some significant damage, however, as the victories and losses section will show you (including two carriers and a battleship).
However, in game terms there is no way they could enter in force, i.e., in a strength 10 unit. You will note that in 2 and a half years of trying, the Germans got 62 U-boats in. This would require some special rules and tweaking of the code to allow for a buildup there over time and account for losses of those trying to pass.
A realted issue that I have been thinking about is Malta and its lack of effect on the game, particularly Axis supply in North Africa. Problem is, with no stacking allowed you can't have both a garrison and air unit there. Malta was a key to the Mediterranean Theater, yet it has no impact in CEAW.