They did not no, the Germans sent some subs over-land to aid the Romanian navy (around 30 I believe) and the Italians were able to sneak ten or so subs into the black sea as well.
Then it's to their credit that they were able to transport these overland from Germany to Sevastopol, having to use 25 trucks.

https://en.wikipedia.org/wiki/Schwerer_Gustav#Schwerer_Gustav

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