Couchettes/ sleepers from Munich to Marseille

I confirmed there is no night train all the way to Marseille
You would have to stop in Milan and that takes 12 hours overnight from Munich.
There is no boat direct to Barcelona from Marseille
There are several cruises from Marseille with other ports of call including Barcelona you just have to find one that ends in Rome
https://www.expedia.com/Cruise-Search?departure-port=marseille-france

Before you plan on all these boats and overnight trains, make sure they exist. As you're learning, with the rise of high speed day trains and budget airlines, these routes are being dropped, with fewer every year.

Rome2Rio is a good way to get a handle on what options exist for getting between two places: https://www.rome2rio.com/

To find train schedules for almost all of Europe, as well as prices for trains that start or end in Germany, use the Bahn (German Rail) website http://reiseauskunft.bahn.de/bin/query.exe/en, following Rick's tutorial: http://www.ricksteves.com/travel-tips/transportation/trains/online-schedules

If you find that you'll have to fly one or more segments (and prepare for this possibility now to avoid disappointment), look for all options on Skyscanner http://www.skyscanner.com/, then book directly on the airline's website to learn all their rules and extra fees (and there will be some).

I don't believe there are currently any scheduled ferries between mainland France and Spain. Grimaldi Lines and GNV both serve some Spain-Italy routes, probably with a stop at Corsica or Sardinia. But all of this long train and boat travel is something I can only image for one person who loves transportation, or is afraid to fly. It doesn't sound right for 5 people with varying interests in long-distance transport. I would prefer to fly and have more time at each destination.