After a slight decrease from severe levels, the mercury is hovering around 40 degrees Celsius in most regions
The accident took place at the Upazila's Protappur village on the Surma River on Monday night, police said.
Superintendent of Police in Sunamganj Harunur Rashid told bdnews24.com that the fire broke out around 9pm when people on the boat were cooking.
The boat was on its way to Chhatok's Bholaganj from Kishoreganj's Etna, he said.
The police were yet to ascertain the identities of the deceased and details of the incident.