Cruise ships carrying more than 12 passengers are required to have a doctor and medical facilities on board. River cruise ships are unlikely to carry a doctor as they are always within reasonable reach of land to access emergency and essential medical assistance.
You will find that you are likely to be well covered if you have any medical problems on a cruise.
However, the same is not true if you have dental problems. Cruise lines do not have dentists and dental facilities on board. If you suffer from tooth ache or other problems, like a crown or filling coming out, the most you should expect is some help with the pain and assistance on trying to locate a dentist in the next port. Many lines used to try to help patch up and deal with simple problems, but the increased insurance risk and chance of being sued if things go wrong meant that they no longer are prepared to provide this.
Before you cruise make sure your teeth are in good condition, ideally by having a check-up. Take a simple tooth repair kit that you can buy from most large pharmacies and drugstores. This will enable you to temporarily fill fillings or stick back crowns if they fall out until you can see a land-based dentist.