I was wondering what was the best company for a day trip down the Rhine river, which city is the best departure point and most scenic. Would need to beable to return to the same departure point on the same day.
If this isn’t possible, then does the Moselle river have any companies that would provide this kind of tour.
This question has already been answered in numerous posts on this forum. Do a couple of searches and you should be able to find a thread that has already answered your guestions.
There are more than one company running boats on the Rhein, but the best known is KD (Köln-Düsseldorf). Here is there website. It contains schedules and fares for their boats.
Most people would agree that the best departure points are either Bingen (Stadt) or Bacharach going down the river to St. Goar. There are trains with hourly service between St. Goar and Bacharach or Bingen. Actually, it is best to start in St. Goar with a train ride to Bacharach or Bingen Stadt and take the boat from there, because by showing a Bahn ticket at the starting dock, you get a 20% discount on your boat fare.
...which city is the best departure point and most scenic.
Both of the big-name companies - K-D cruises and Bingen-Rüdesheimer cruises - offer what is essentially the same cruise through the most scenic part. K-D calls it the Loreley-Tour. B-R calls it the Loreleyfahrt. Here is a map of this cruise route:
http://www.bingen-ruedesheimer.de/wp-content/uploads/2012/02/Loreleyfahrt.png
For many years, both companies have begun the cruise in Rüdesheim and covered the river between there and St. Goar/St. Goarshausen. This stretch is the most scenic and the one in greatest demand. (K-D boats also continue north slightly further to Boppard, and a smaller number go further north to Braubach and eventually Koblenz.) The boats return to R'heim, so it is possible to reverse the cruise direction - but it takes much longer that way because of the current.
Where are you planning to stay before the cruise? If you are on the east bank of the Rhine, the Rüdesheim side, then start in Rüdesheim. If you are on the west bank, start in Bingen instead (which is just across the river basically.) Whichever company you choose, at the north end, you can get off either in St. Goar or St. Goarshausen - whichever is more convenient for your plans that day. (If you plan to visit Boppard anyway, then go with KD and the slightly longer cruise.)
If you are returning to Rüdesheim or Bingen, take the train. Returning by boat is very time consuming and repetitive.
About starting a cruise in Bacharach rather than Bingen or Rüdesheim... I would not choose to do so. Have another look at the map. You will miss half of the most scenic section if you do, and you'll have only about 45 minutes on the boat before you hit St. Goar.
The only person that recommends starting your ship ride in Bacharach going to St. Goar is Rick. You miss 3/4ths of the castles starting so far down the river. No idea why he skips so much of the Rhein as well as recommend that others skip it too.
My personal recommendation is to start in Eltville and go to St. Goar. Train back and get off in the towns you like. Eltville is one of the most charming towns along the Rhein. Only one ship a day though.
Next best is to start in Rüdesheim or Bingen.
Are you staying in Frankfurt?
No matter which starting/ending point you pick ( I like Bingen and Boppard ), just take the ride downstream ( north ) and take the train on the return trip. Stop at any town you want to on the return, such as Oberwesel and Bacharach.
Eltville is a nice place to visit, but for scenic river cruising the landscape there is largely flat until just beyond Rüdesheim/Bingen.
https://pbs.twimg.com/media/DcwAV_8WsAA13qE.jpg
Scene near Bingen: http://www.bingen-ruedesheimer.de/wp-content/uploads/2012/10/LZ1B0487-als-Smart-Objekt-1.jpg
Also, it's an extra hour on the river from Eltville to R'heim... so the Eltville - St. Goar KD cruise fare is €38 (vs. €20 from R'heim.) So if you are visiting (or staying) in Eltville and want a cruise, I'd take the train to R'heim (€5) and board there.
Or maybe KD is passing out free chocolate croissants and coffee as you board in Eltville??? That might actually be a good marketing strategy for that part of the river.
Well, I actually find the scenery to be very pretty around Eltville. The vineyards stretching up past the town, lots of greenery along the river bank. Visiting the town itself before getting on the ship is worthwhile too.
"Visiting the town itself before getting on the ship is worthwhile too."
Day visitors would need to conclude their Eltville visit by 10 am to catch the last northbound boat of the day.
https://www.k-d.com/fileadmin/kataloge/KD_Timetable_intern_2018_web.pdf