score:6
The Twin City Liner schedule could perhaps be laid out more clearly, but careful perusal shows that their season runs from 25 March 2016 to 30 October 2016.
Another option is the train; this is faster (1 hour), cheaper, and runs all year round, but of course isn't quite the same experience as a river cruise :). Here is ÖBB's leaflet about the Vienna-Bratislava service, including a timetable.