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Thank you. What about subs?

Possible, but a submarine usually relies on the fact that it's a small sub in a big ocean for safety. The Black Sea does not qualify as a big ocean, it would be hard to remain undetected there. On the other hand, a sub is a small chess piece, unlike a carrier, therefore easier to risk.

Any ship that passes through the Turkish Straits on the surface will be seen by Russian spies. There have to be some watching ship traffic at all times. And a sub is unlikely to pass through under water, since it would be so dangerous. The Straits are shallow and winding, and there's a lot of surface traffic. Easy to either ground the sub, or hit a surface ship, both of which would be bad. (I'm not even sure the straits are deep enough for a submarine to pass through without surfacing.) But the word "unlikely" does not mean the same thing as "no possibility".