train from munich to passau

Munich to Dachau is by suburban train (S-Bahn), taking about 20 mins. There are buses to take you to the concentration camp, or you can walk, which is around 45 mins. Munich to Passau is by regional train (RE), taking around 2 hrs. There will be a taxi to get you to the landing.
Both trains don't come with seat reservation, meaning they can never be sold out. Tickets can be bought at the railway station, at the window or from a machine, whatever you prefer. Look at the train planner of the Deutsche Bahn for the schedules.
Depending on the departure time of your cruise it can be a slow-paced day or a stressful affair.

When do you need to get to Passau to catch your boat? There might not be enough time if you go to Dachau first.

The recommended connection between the Dachau Bahnhof and Passau goes through the Munich Hbf. There is an S-Bahn every 20 minutes from Dachau Bahnhof to Munich Hbf and a regional connection between Munich Hbf and Passau every hour at 24 minutes after the hour. The trip takes just under three hours.

You probably want to check your baggage somewhere and retrieve it before you start to Passau. There is no place to check you baggage at the Memorial, and the lockers at Dachau Bahnhof are limited. I wouldn't want to get out to Dachau Bahnhof and find that all of the lockers there are taken. So check your baggage at the Munich Hbf and get it when you get back from Dachau.

English tours at the Memorial start at 11 AM and 1 PM and run 2½ hours. Depending on when you have to be in Passau, you probably don't have time to take a guided tour, but the Memorial opens at 9 AM, so you might be able to just get there when it opens and do the tour on you own (self tours are free, audio guides can be rented) and still get to Passau on time.

BTW, this URL is the one I prefer to use to get German Rail schedules and prices.

Don't underestimate the strong emotional effect that a visit to the Dachau KZ can have on one or more of your group...

This can affect the fairly long train journey.

Since the arrival at the boat should be a happy time might you think of rescheduling some other day or activity to not have the KZ on the same day as the beginning of your boat experience.

I know that both my wife and I were quite moved by the experience - especially the shoes.

Remember that the whole group moves at the speed of the slowest (or most emotional) member.

Any children in your group of 6?

Maybe true of younger visitors, but I, having been born in the shadow of WWII, grew up hearing ad infinitum about the holocaust until I essentially became immune to it. It was actually a great relief to me when I visited Dachau to find that, although a minority of people there were worked to death, it was a work camp, where prisoners actually did work and most were eventually released, not a death camp, where they were brought there with the sole intention of killing them.

I might be more affected by a visit to Auschwitz.

Assuming that you would want to arrive at your cruise boat not later than 6pm you'd have to leave the memorial not later than 14:06. That would give you a break of ca 45 min. in Munich Hbf to pick up your luggage and to grab some food (the Munich - Passau train is a regional one, no food and no drinks available, so stock accordingly). So: Memorial dep. 14:06 - Dachau Bf dep. 14:25 - Munich Hbf. arr. 14:37. Munich Hbf. dep. 15:24, Passau arr. 17:37.

In the morning, if you take not the S-Bahn from Munich Hbf but the faster 9:27 RB (destination Nürnberg) you would arrive at the memorial by 9:48 (change to bus #726 at Dachau Bf). That would give you four hours for visiting, propable more than you want (or are able) to spend there. If you decide on taking that train you could use a Bayern Ticket for the entire trip Munich - Dachau - Munich - Passau; BT's are valid from 9am, no restrictions on weekends, children under 15 yo travel for free (buy a ticket for the appropriate number of adults from a vending machine and write down on it all names of adults; don't board the train without ticket, the fine will be hefty). Price is €25 + €6 for each additional traveller.

Please check where your boat docks in Passau. It might be near the station but it might be in Passau-Lindau as well, which is 6km away. You can look up the distribution of docks for the current month here (in German, scroll down to "Liegeplatzeinteilung").

Being that the group is 6 in number, you will need 2 Bayern Tickets to do this, as the Bayern Ticket maximum is 5 persons. Still at 74 EUR ( 37 + 37, or 43 + 31), for all the transport for that day, it's a bargain.

Being that the group is 6 in number, you will need 2 Bayern Tickets to do this

True, unless four of the party are one of the other two adult's children or grandchildren under 15, or if one or more of the travelers is a child under 6.

Lee

German joke about Dachau

Two men meet [on the street].

“Nice to see you’re free again. How was it in the concentration camp?”

"Great! Breakfast in bed, a choice of coffee or chocolate. Then some sport. For lunch we got soup, meat and dessert. And we played games in the afternoon before getting coffee and cakes. Then a little snooze and we watched movies after dinner.”

The man was astonished: “That’s great! I recently spoke to Meyer, who was also locked up there. He told me a different story.”

The other man nods gravely and says: “Yes, well, that’s why they’ve picked him up again.”