The Russians think of the Black Sea as a Russian lake:
The Kremlin wants the rest of the world to believe that it has a special role and set of rights in the Black Sea. Too many Western policymakers fall for this. In truth, the political, economic, and security development of the region is important in its own right.
Russia’s push on the Black Sea dates from the Tsarist era. During the Cold War, all littoral territory apart from Turkey was part of either the Soviet Union or its Warsaw Pact allies. Now Russia has a legal claim to only 10 percent of the Black Sea coast. The other littoral states, with a population of 150m, are independent, democratic, and seeking their own economic development and security. Three are NATO allies and the others are NATO partners.
During the Cold War it pretty much was. Now it isn't and the Russians don't like it:
A Russian fighter jet intercepted two U.S. reconnaissance aircraft and a refueling tanker as they flew in “neutral” airspace over the Black Sea, the Russian defense ministry said in a statement.
The intercept was at least the 12th this year involving U.S. and Russian aircraft worldwide, according to a tally by Stars and Stripes. Most have occurred in the past month.
I think that Russia is more touchy about the Black Sea ever since the Russians illegally captured and annexed Crimea from Ukraine in 2014. Crimea is now a staging area for Russian operations in the eastern Mediterranean Sea (and I think that Russia intervened in Syria to justify the price of taking Crimea).
Although my feeling is that while NATO should take advantage of geography to inflict casualties on Russian forces in case of war, the Black Sea is a potential distraction from the main Baltic Sea front threatened by Russia.
UPDATE: America is providing small boats with heavy firepower to Ukraine, which lost most of its ships in 2014:
With both the [American] Mk VI and Guzra-M forces at full strength Ukraine will have 36 modern gunboats, all armed with 25mm or 30mm autocannon RWS (remote weapons stations) and a pair of modern anti-ship missiles. This will give the Russians something to worry about even if all those gunboats will not even the naval balance in the Black Sea. Not that the Russian Black Fleet is a major force.
Combined with NATO forces, the Ukrainians will be able to join in the turkey shoot.
And I have one suggestion for the Ukrainians: Naval mines. Lots of them deliverable by air or sea to make Sevastopol unusable in war.
