Awful Question....

[unmerged(19908)]

You could use the trick used by the Turks during the siege of Constantinople. There was a big chain blocking the strait so the Turks portaged their navy overland during the night and just redeployed in the water on the poorly defended side of the city.

 

[DarkLight]

A few U-Boats managed to enter the Med through the Strait of Gibraltar. They did it submerged using the prevailing current to carry them through silently. However, once in the Med, they were stuck. They couldn't get out the way they came in because they couldn't make sufficient headway against the current while submerged.