The incident occurred when the boat set off for Pavitra Sangamam at Ferry village near Vijayawada from Bhavani Island, the police said.
"Many of the tourists got trapped under the overturned boat and died of suffocation", said one of them.
Most of the passengers were said to be members of the Ongole Walkers Club in Prakasam district while some were from Nellore who were on a visit to Vijayawada.
According to the AP State Disaster Management Authority, there are two NDRF teams with 30 members each, 45 members of the State Disaster Rescue Force and 12 rescue boats now working on the rescue operation.More news: 'I would NEVER' call Rocket Man Kim Jong Un 'short and fat'
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Andhra Pradesh DGP Sambha Shiva Rao confirmed recovery of 16 bodies so far in the tragic incident. We are in the process of identifying the victims.
Local fishermen at Ferry swung into action immediately and rescued 15 passengers. YSR Congress Party President YS Jaganmohan Reddy also expressed his grief, and said that the official machinery should speed up the rescue operation. They inquired with the APTDC regarding the availability of a boat, but were told that the boat service had been stopped due to poor light in the evening.
Around 20 of the roughly 35 people on the boat were rescued and search operations were continuing, India's National Disaster Response Force said on Twitter.
The incident took place at Ibahimpatnam mandal in Krishna district.