We took a river cruise when we visited Budapest in June, 2015. There are several river cruise companies to choose from; we decided on Legenda because it offered discounts for Rick Steves guidebook (we only had ebook but asked for it anyway, and the cashier gave it to us without betting an eye). Their website on evening cruise is here and their Google Maps location is here.
To this date, we are not sure why a fireworks broke out during our cruise (our story, of course, is that they knew we were in town). We were so mesmerized we forgot to ask for our free drink. Enjoy the video!