Under the Montreux Convention, Turkey has declared that as there is a "live" conflict, no military vessels are allowed through the Bosporus/Dardanelles Straits. The only exception is if they are normally stationed in the Black Sea.
Apparently Russia tried to send 4 vessels through and was refused permission for 3 of them (the 4th could claim normally to be stationed in the Black Sea but since it would be going in unaccompanied, decided it no longer "needed" to get there).
So any bombardment/invasion from the sea by the Russians would have to be from the ships they currently have in the Black Sea and they can't be replaced 
So it's a "one chance" naval advantage. No doubt (and I'm no military strategist) that's why they're so desperate to link up the land bridge before making a land and sea assault on Odessa.
On the other hand, it also means that no other country can send in ships to the Black Sea to support Ukraine 