It's unclear how many people are missing, but an unidentified military intelligence officer told Reuters at least 50 people had drowned after a boat - possibly one of six leaving Pauktaw - struck rocks and sank.
An spokeswoman for the United Nations' Office for the Coordination of Humanitarian Affairs in Bangkok said a large boat tugging two smaller boats was involved.
"We understand that yesterday evening they went out with the approval of government officials," Kirsten Mildren said. "This was part of an official government evacuation plan although the boats were not government boats. They were moving from a low-lying area to a safer area."
NASA says Mahasen could "reach hurricane force" by Wednesday, and the UN has called for the evacuation of tens of thousands of Rohingya living in makeshift camps, where risk of flooding and tidal surges are high.
Sectarian violence that began in Myanmar last year has displaced about 140,000 people, leaving them exposed to severe weather like Mahasen, currently gaining force in the Bay of Bengal.
"If the government fails to evacuate those at risk, any disaster that results will not be natural but man-made," said Brad Adams, Asia director at Human Rights Watch.
Violent and deadly clashes between Rakhine's Muslim minority and Myanmar's Buddhist majority erupted last June in what Human Rights Watch has since called "a campaign of ethnic cleansing against Rohingya Muslims in Arakan State."
Here's an infographic via AFP's John Saeki:
The UN had, according to AFP, previously warned Myanmar (also known as Burma) against the coming monsoon season.
"The government has been repeatedly warned to make appropriate arrangements for those displaced in Rakhine state," said Isabelle Arradon, deputy Asia Pacific director of Amnesty International. "Now thousands of lives are at stake unless targeted action is taken immediately to assist those most at risk."