The CDC recommends that all people avoid travel on cruise ships, including river cruises, worldwide, because the risk of Covid-19 on cruise ships is very high. It is especially important that people with an increased risk of severe illness avoid travel on cruise ships, including river cruises.
Cruise passengers are at increased risk of person-to-person spread of infectious diseases. Outbreaks of Covid-19 have been reported on cruise ships.
Passengers who decide to go on a cruise should get tested...
https://www.workboat.com/news/government/cdc-says-not-to-travel-on-riverboat-cruises-cruise-ships/