I know that in the actual war Turkey closed it to combatants but it would be interesting to have an option as Russia to pressure or bribe Turkey to allow shipping or perhaps shipping + warships. Maybe the shipping-only option could have a higher chance of succeeding. That might allow Sevastapol to survive as a fortress city (think Leningrad) even if the port to the east falls.
I wonder if submarines could actually slip through? In the game they can't.