http://www.reuters.com/article/2014/03/07/us-ukraine-crisis-usa-warship-idUSBREA260Y820140307
Also it means that you'd pass through/close to our waters ffs!
But the Bosphorus isn't safe enough for subs to go through, I think... I mean I'm not an expert but it's probably not deep enough for submerged passage.
You can traverse the Bosphorus submerged, but there is a chance you would scrape some paint in some sections. The Soviets chose the Black Sea as it is very defensible but giving easy access. It does create a very easy choke point though