The incident occurred when 22-year-old Meesar Ali, the boatman, was taking a group of tourists from Mallital for a tour of the lake, he said.
Reportedly, Ali was ill for the past few days.
Rescue and search operations have been started by the water police and the State Disaster Response Force (SDRF), the official said.
The operations are being supervised by the Nainital Superintendent of Police, Harish Chandra Sati, and other senior officers, he said.
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