It’s extremely rare for anyone to get seasick on a river cruise. First, there are virtually no waves on a river. Sure, it can get windy, but waves and rocking are almost non-existent. If your river ship is tied up in port at night, you might experience a slightly gentle sway now and then when a cargo boat goes past.
And speed? Hardly. River ships move at only a fraction of the speed of ocean cruise ships. Slow enough in fact, that you can read the nautical mile marker signs along the river banks. And with the shoreline always in sight, there’s always something to watch to get your mind off any queasiness you might encounter. But it’s rare that anyone feels ill. In my ten years of river cruising I’ve never seen anyone wearing a seasick patch nor mention a bout of mal de mer.
There are some places, like on the Volga in Russia or parts of the Mississippi, where the river seems to open up and you feel like your on a lake rather than a river. But it is only for a few minutes and then the river narrows again.