The fire was reported aboard a boat almost directly under the bridge at 11:53 a.m., Fire Marshal Lewis Broschard said.
No injuries have been reported, and anyone who may have been aboard the boat escaped safely, he said. The boat sank at about 12:40 p.m.
Broschard said the fire spread to part of a pier, and that firefighters have controlled the blaze. As of 12:50 p.m., firefighters remained at the scene.