The sinking of the Black Sea Fleet would be a huge escalation and a declaration of war, not to mention not a smart thing to announce your plan in advance.
To me it's the same as when Russia makes big threats. It's a war of words but with no plan for actual action.
Edit:
It seems to me that the US are trying to corner Russia into escalation and warning of potential Russian escalation as the same time.
Russia has made a mistake in invading Ukraine but the US seem to be the force behind making the situation worse and worse since, possibly with the objective of regime change in Russia. Considering the US's track record that should make everyone worry.