The boat capsized late on Saturday and no survivors have been found since Sunday afternoon, when Guyana authorities announced they had rescued 67 people, including 15 children.
The captain and several crew members are now in police custody, Public Works Minister Juan Edghill said at a press conference late on Sunday.
"Search and rescue is still in progress," Edghill said.
"So to the families that are out there wondering about their loved ones, we are doing everything that is humanly possible to find them."
The MV Barima, which had 116 passengers and 17 crew members, capsized off the North Atlantic coast, approximately seven nautical miles from shore, while travelling from the capital Georgetown to the village of Port Kaituma, authorities said.
Air traffic control received a distress call from the ferry around 11pm on Saturday, triggering a search and rescue operation.
Edghill said early responses were hampered by the darkness and lack of vessels with scanners that could search for survivors, leaving rescuers reliant on eyesight and sound.
Companies in the oil and gas sector have since deployed vessels with scanners and the government has activated divers to enter the ferry to search for bodies, Edghill said.
Authorities have also expanded the search area to 1040 sq/km.
Edghill initially insisted on Sunday afternoon that there was no sign of "neglect" on the part of the ferry operator. But on Sunday evening, he said the captain and several other crew members had tested positive for marijuana.
In addition, a number of people who were rescued were not listed on the ferry's manifest, suggesting inaccuracies in the ferry's passenger list.
Guyananese Prime Minister Mark Phillips said an investigation had been launched into wrongdoing involving members of the police force, defence force and maritime administration department.
"Let me be absolutely clear. Were negligence, misconduct or criminal wrongdoing established, those responsible will face the full force of the law."
