If you mean the station at Schwedenplatz, then you just need to get down in the metro, take the green U4 branch and go to the Wien-Mitte station.

Wien-Mitte railway station and Wien-Mitte Vienna bus station nearby has Landstrasser Hauptstr. 1b. Nearby are the 3rd and 4th lines of the Vienna Metro, the Landstrasse metro station. In addition, nearby is the Ringstraße, well connected to the intracity transport network - not only the metro but also the tram, by the city bus.

Wien-City River Station is located at Schwedenplatz, Marienbrücke. According to Vienna's public transport map, you need to go from line to line, but the path is not far and will take about three minutes.

You can overcome this path by land transport, especially since all Vienna's public transport (buses, trams, high-speed city trains, metro ) consist in a single transport system Verkehrsverbund Ost-Region (VOR), which makes it possible to travel the same ticket in all modes of transport. For example, it is possible to make a trip from Löwengasse (nearest to Wien-Mitte) to Schwedenplatz in just 8 minutes by trams of Wiener Linien (1 line).

However, you can overcome the path of 2, 7 km and walk for just some half an hour (if, of course, follow the light). The taxi will handle this in 5 minutes and about 8 EUR.