Are there different travel times for sailing upriver and downriver?
And how does the traveling time for river travel compare to going on foot or by horse?

I just defeated my first band of STEAM fan boys...I mean Satan worshippers and a demon. One of the dying cultists cursed us until 21 March when something will happen NW of Teschen. Teschen is in the extreme east and somewhat to the south. The date is currently 18 November and my party is located near Leer in the North-west.
I wonder if I have a chance of making it in time. I'll get 7 Florins in Hamburg and then probably have enough money to buy fast horses for all.

Oh well, I reached Teschen at the end of January, after riding from Hamburg.

I find it's rare where the game doesn't give enough time to get to a location by a certain date. Assuming you go directly there.

I tested out travelling between Hamburg and Magdeburg, by boat and by foot.
By boat: 4 days
By foot (on open road): 10 days.
On foot, there were only a few hours of delays, (I'll guess 5). So I wasn't spending much time being stalled by events.
Like I thought, boats are very fast! If you were curious.

Did you notice any difference between upstream and downstream?

Boats are 3 days for a single destination (there is also a 0-12 hour waiting period for setting sail, and if the boat is attacked, there is an unknown delay)

If you stay on the boat when it arrives, the captain quietly signals his boat to unfurl its wings and fly low along the Rhine like a mighty water dragon. Each next destination is only 1-3 hours.

Destinations change at hour 0 of each day. Sometimes you can just cycle through 2 active destinations until the day changes, and then continue further up/down river.

On land, roads are obviously faster than other terrains. Praying to Saint Christopher has no effect on travel speed (despite documentation). Neither do horses. They still help you through encounters though.

Saint Vitus teleports you to the nearest city while you are in the overland mode. He is our best friend for land travel.