It's a violation of the Montreu(sic) Convention for submarines of non Black Sea powers to transit the Turkish straits, and the Turks are pretty anal about that these days. Besides, the straits are narrow, not deep, heavily traveled, and with tricky currents. The Bosporus bisects the city of Istanbul, formerly Constantinople. Hard to sneak much of anything through there. A sub might make it through at night and at periscope depth without being seen, if the Turks looked the other way that is.
Whatever you say! I am not going to loose lip and sink ship.