Arriving early is worth more than anything
The benefit is not only calm: you can find your berth, verify the ticket, use the toilet and still take a look at the confluence from the bank. Arriving at the last minute often means you are already tired by the time you board.
Which side to stand on
If you want to face one direction, first see which side the main sights are on. But the more practical advice is that the boat turns and both sides get their turn — do not fixate on one side. The return leg often has the better angle.
A jacket is a hard requirement
The river is windy and feels much cooler than the shore at night — summer included. A light jacket adds more comfort than its weight suggests.
Phone battery
Check-in codes, photos and contacting your group all depend on the phone. Cold plus long shooting drains it faster than expected. Bring a power bank, or charge fully before leaving.
Do not overfill the schedule
Leaving half an hour of buffer before and after is more useful than squeezing in another sight. What usually spoils the experience is not "missing one place" but "rushing the whole time".