I know. They however boasted their ships and men would prevent a crossing. Russian hyperbole notwithstanding.Unless you have an overwhelming technical or numerical superiority, putting warships into enclosed waters is a bad idea. Some idiot journo or internet admiral most probably thought it a good idea.
It more than a stone's throw from the Dnepr liman (estuary) to the Perekop (istmuth) or Sivash (shallow wetlands) which separate Crimea from the "mainland", hence it is a certain jump from "crossing the Dnepr" to "invading Crimea".