I will be in Vienna in late September and plan to do the OBB train/boat tour to Melk. Is it safe to wait until the last minute or do I need to buy in advance? Would like to wait to see what the weather will be.
My recollection is that we bought the tickets day-of, for the same reason you stated. There are several boats and many trains.
We're doing the same thing in early September. My understanding is that there are only a few boats, so check the schedules carefully and even though there are periodic trains, best to get an earlier one to maximize your time in Melk.
It is no problem to book the day before.
Our visit to Melk and the Danube Valley was different. We booked the Viator tour. We took a bus from Vienna to o to a point along the Danube where we boarded a boat for the remaining trip to Melk. The trip on the boat was long enough for be served a very nice lunch and see some of the beautiful sights along the river. We then visited Melk and then returned to Vienna by bus. 8hrs overall. No photography inside Melk (at least on our visit).
I have purchased the combo ticket or our trip in September. The ticket is not for an exact boat or train but for a period of time, in my case 9/16-29.