My husband and I take the ferry at least once every year for visiting Newport in summer, when traffic and parking are unbearable. For $18 an adult can get on and off at any of the 5-6 stops along it's route and back to either Jamestown or Newport, wherever you departed from. The ferry itself is small, holding perhaps 30 passengers. You sit down close to the water. People with mobility issues would have trouble boarding and unboarding. Stairs and a step stool must be navigated. It is not wheelchair accessible.
This is an excellent activity for those with children. You can stop and tour a lighthouse or an historic fort (Ft Adams0. You pass quite close to a number of sail boats out on the water as well as yatchs and mega yatchs at anchor in Newport Harbor. For adults there are inexpensive drinks available. The ferry allows dogs aboard.