Twenty-five years ago, when I lived in Vienna, I would take visitors to Melk on the train from the Westbahnhof, tour the Melk Abbey, then return to Vienna on a boat that arrived back in Vienna around 7:30 PM (at Schwedenplatz? memory fails me!).
I am about to return to Vienna after 25 years, and looking at the boat schedules, I do not see any of them traveling from Melk to Vienna. Instead all their routes terminate in Krems, and from there one must take a train back to Vienna.
Does anyone know of a ship line that carries passengers from Melk all the way back to Vienna?