As I know, you definitely don’t need the Russian visa if you are not going to leave the ship during stay in port. So you need to worry about it.
Russian embassies can help you to obtain Russian visa due your unusual situation, so you can try it all the time – we are all the people, and you always can try to communicate with embassy. Also you should note what 2011 year is a start year for the 2011-2018 tourism campaign, and you have great chance to solve your problem.
The nearest shipment from Kerch to the Poti will be tomorrow, so you shouldn’t be afraid for this ship is not available. Also I’d like to note that this route is direct and has no stops in Russia.