Omsk – Salekhard 2783 km, 6 days. Currently as of 2018. River boat. I don’t know if it takes cars.
Some sea or ocean crossings that are longer than 1000 km, as the crow flies (current as of 2012):
Even longer: The river boat from Omsk to Salekhard (Siberia, Russia) runs for 2783 km. If I’m reading the timetable correctly, the journey takes 6 days (and a connecting ferry from Salekhard to Antipayuta is another 2 nights and 715 km). I don’t think it takes cars, but I’m not entirely sure about that, as the only other way for people in some communities along the river to bring a car would be by winter road, so maybe there is a provision for that.
There may be other very long river boat journeys in Russia, in particular in Siberia. Siberia is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, but that’s just peanuts to Siberia¹.
¹After Douglas Adams.
Credit:stackoverflow.com‘