Hello! I could use some help determining the provider for the Melk River tours. Should I purchase tickets ahead of time?
No need for advance purchase. Here is one operator site:
If you come from Vienna you can also purchase a ticket for thr train to Melk and boat to Krems, train back to Vienna
For a day-trip to the Danube I recommend catching an early train to Melk, tour its abbey, eat lunch, and take an afternoon trip on the river from Melk to Krems. From Krems, catch the train back to Vienna. The Austrian railway sells a convenient Wachau Kombiticket, which includes the train trip from Vienna to Melk, entry to the Melk Abbey, a boat cruise to Krems, and the return train trip to Vienna. How ever you choose to visit this area I don't see a need to book things in advance since the ferries/cruises between the towns or from Vienna run quite often.