At least 16 people drowned and many went missing when an overcrowded trawler capsized in the Muriganga river near Sagar Islands in South 24 Parganas district of West Bengal on Sunday.
The authorities fear the toll could rise.
Three trawlers were sailing to Kakdwip in South 24 Parganas from Hijli Sarif in East Midnapore district when one of them capsized.
"Sixteen bodies have been found and more than 50 persons rescued while many of the bodies were swept away by swift current in the river," Additional Director General of Police (Law and Order) Surojit Kar Purokayanstha said.
"There is a possibility that the death toll will increase because the trawler was overcrowded," South 24 Parganas Superintendent of Police L N Meena said.
The Coast Guards, marine and district police were working on a war-footing but it would be difficult to carry on rescue operations in the sea after dark," Meena said.
The rescued people have been rushed to Kakdwip hospital for treatment, Meena said.
West Bengal Minister for Sundarban Development Kanti Ganguly, who was overseeing rescue operations, said "rescue operations are proving to be difficult because of the current and movement of ships."
District Magistrate of adjoining East Midnapore Archana Meena said marine police and Coast Guard with hovercraft had been sent to assist in the operations.
A Defence ministry spokesman said navy divers from Eastern Naval headquarters at Visakhapatnam were rushing to the spot.