The vessel, Pelican-1, caught fire in Harbaria around 6:30pm on Friday, Divisional Forest Officer (East) Md Saidul Islam told bdnews24.com.
Mongla Coast Guard (East Zone) Staff Officer Lt Commander Md Rahatuzzaman said, "Work to douse the fire is under way. The Navy is also at the scene. All the tourists have been evacuated."
The Fire Service as well as fire-fighting units at Mongla Port have also been called in to help, he added.
Holidays Tours and Travels, which organised the tour, said 27 tourists were on board the vessel.
The agency's owner 'Babu, who did not provide his full name, told bdnews24.com the launch had sailed for Harbaria in the morning with permission from the authorities.
"It anchored at Harbaria Camp Pier of the Forest Department. It was scheduled to start its return journey in the evening, but most of the passengers were not on board when it caught fire," he said.
Babu said all 28 cabins of the three-storey vessel had been damaged in the fire, but no one had been injured.
He could not say what caused the fire, but suspected it had originated from the kitchen of the launch or a carelessly thrown cigarette butt.