Thank You for your kind words darrelhilton - much appreciated. I believe the above poster mistakes the old soviet era hydrofoil that runs between Vienna and Budapest with the much newer, faster and more comfortable Twin City Liner ( website quoted above ) that runs regularly between Vienna and Bratislava. The twin city liner has a limited draft, a catamaran, that allows it to enter the Danube Canal and dock in city center at Schwedenplatz area. It is relatively quiet, nice seating and enjoyable to ride.
The trip about 75 minutes-
The Hydrofoil only is able to dock in the Danube - this at Handelskai not in city center.
It needs more water depth to navigate. These boats have had some modernization but are still noisy, relatively slow and not a great ride. This trip a long expensive and not very comfortable time.
My personal opinion- Bratislava is not that interesting - just a ok small city center area- the remainder of the city makes one think of the partial recovery from the Cold War era.
With so much to see and do in Vienna and environs, your limited visiting time better spent there.