Rhine river cruise

I have never heard of that company. Tried to find their pricing on their website. The tour does not look like a good tour though. See 5 castles? If you take the ship from Bingen or Rüdesheim down to St. Goar, you will dozens of castles and the ride only lasts a bit under 2 hours.
If you are staying in Mainz, just take the short train ride to Bingen and get on your ship there. Or if you have a car, park your car there. Bingen is a short ride from Mainz on the train. If you are planning on drinking some wine on your trip, take the train.

The KD ships sail on Monday. What website are you looking at?

"I wish now we had booked our hotel in Rudesheim."

Mainz on its own is quite a nice city, R'heim nice too but much much smaller - a wine and tourist town. Very different places! But Mainz, if that's your final choice, will still be OK for doing a river cruise. I have never heard of this tour company either. IMO you should do as Jo says... and as most smart visitor who wish to cruise the Rhine from Mainz should probably do... catch the train to Bingen and then catch the ship to St. Goar. This is the most essential part of the river. St. Goar is where 3 of the 4 daily K-D boats terminate. The Bingen-Rüdesheimer company offers 2 Rhine cruises - both of them cover the same exact segment of the river. It's the most popular part, obviously, and for good reason:

https://i.pinimg.com/736x/ca/e1/50/cae1500f42cf03c44bea2da512d659a0.jpg

The morning boat from Mainz covers this same segment - it's the SAME BOAT that leaves Bingen at 11:15. But the first two hours of cruising between Mainz and Bingen will offer you very little great scenery by comparison. There's only one boat per day because very few people are interested in cruising this part and/or paying so much more to cruise this part AND the more scenic Bingen > St. Goar part.

And you are absolutely right about the schedule, which clearly states that this boat runs only Tue - Sun from MAINZ.

So doing your cruise on a Monday might actually have saved you some time and money from your Mainz base!

So... Travel by regional train from Mainz > Bingen Rhein Stadt (station name.) You will need a train ticket for this day too... for the train to Mainz plus the train back to Mainz from St. Goar. A day pass called the Rheinland-Pfalz ticket covers 2 adults for €32:

https://www.vrminfo.de/en/tickets/tickets/leisure-tickets/rheinland-pfalz-ticket/

Show this train ticket at the KD dock and you get a 20% cruise discount.

1-way cruise to St. Goar = 2x €25 = €50 subtract 20% discount = €40 + €32 train day pass = €72/day cruise outing.

AFTER YOUR CRUISE... stuff to do...
- Rheinfels castle tour in St. Goar
- Use the train pass on the way back to Mainz to stop off in Oberwesel (town wall walk) and Bacharach (handsome half-timbered buildings.

Thank you very much, this has been helpful.