Edit: rather than just downvoting, why not consider that the Danube exists and is plenty of patrol boats/frigates based up and down it by Ukraine friendly nations.
My guess is 'warship' is doing a ton of heavy lifting in this article. We are most likely talking about something navigable by the rivers routinely used for cargo that move through Europe into the Black Sea.
Ukraine has successfully turned waterbikes and rubber dinghies into warships, capable of taking down attack helicopters, fighter jets, and capturing all the rigs off the coast of Crimea.