The boat owner, another person and a dog were launching the boat from Lake Meridian Park at about 9 p.m. when it somehow caught fire, according to Ryan Montero with the Kent Fire Department.
The boat was still at the dock when the fire started, so the two people and dog were able to jump to safety.
Fire crews arrived at the lake a short time later. They controlled the blaze, but allowed the boat to burn, Montero said.
The boat was severely damaged, but the fire didn't spread to the dock or any other vessels. Investigators don't yet know what caused the fire.
The State Department of Ecology and the Coast Guard will advise the fire department on how to handle any possible pollution issues.