"It's an iconic piece of history ... a lot of history," police Chief James Craig said.
However, Ste. Claire owner Ron Kattoo said he hoped to rebuild the boat if possible. He called the fire and damage "heartbreaking" as firefighters on the dock doused the flames with hoses.
Usually loaded with passengers, the Ste. Claire and SS Columbia travelled for decades between Detroit and Boblo Island in Canada, about 24 kilometres away.
The three-story, 197-foot Ste. Claire was built in 1910. The Columbia was launched in 1902. The boats made their last runs in 1991, and the amusement park closed two years later.
Both steamships were designated National Historic Landmarks in 1992.
The Columbia was sold and moved by tugboat in 2014 to Toledo, Ohio, for repairs before heading on to New York state, where it was to be restored as a floating museum.
- AP