I just return from a small vacation and see this posting about one of my favorite places- the Danube, especially in the Wachau.
It is not needed to pre book .
As you probably already see in the tours and websites mentioned numerous offerings.
I would not take the boat from Vienna to the Wachau or Melk.
The trip from Vienna to the first nice place, Krems is not really interesting. It also takes a long time.
Taking the combi ticket - train Vienna- Melk , Abbey tour , then the ship to Krems or Dürnstein, then train back to Vienna is much better in my opinion.Price for all above ( also a nice museum in Krems) is 42,40 euro.
Buy your ticket at the Westbahnhof ( train station )
The Brandner ship line also offers tours.
http://www.brandner.at
in english too.
these are similar to the DDSG.
You can of course create your own trip buying a train ticket and ship ticket separate.
Perhaps train to Melk ( http://www.oebb.at for schedule) Melk, the boat trip to Dürnstein ( this part of the danube Wachau is the nicest view- some time in Dürnstein to sightsee- then the local train from Dürnstein to Krems for an easy transfer to Vienna.
Or I am often in this area in the summer- could always meet you- show some of the special places including some very nice wineries- we can stop for a glass of good white wine.
A newly announced trip to this area may be interesting to some. It includes an interesting stop at a very good wine exhibit with wine trasting and also boat trip.
It will be offered each Sunday beginning in May 6 to October.
It includes for 34 euro ,bus trip from Vienna ( by State Opera) to the town of Langenlois and a visit to the Loisium ( wine exhibit in old cellars , with audio guide, gratis wine tasting, a, boat trip from Krems to Spitz in the Wachau and return by bus to Vienna.
I find more and post it here.
I have taken U S friends to the Loisium. They enjoyed this very much,
the wine too.
Trip starts at 10 return by 6 PM. a very interesting day.