"We have recovered more dead bodies. The death toll has now risen to 17, two women were among the dead," a senior rescue official told AFP, adding the search operation was ongoing.
Local government official Shaukat Ali earlier told AFP that 22 people had been rescued. Ashiq Malik, medical superintendent of the nearby Nishtar Hospital, said the bodies of two children and bridegroom Zahid Ali, 27, were brought there.
Most passengers were members of a wedding party that had requested the use of the rescue boat to take them to a Valima.
"I saved my life by holding on to an electricity pole," distraught bride Mashal told private news channel.
President Mamnoon Hussain expressed "profound grief and sorrow" at the incident in a statement released by his office. Ali, the official, said an enquiry had been ordered. Jam Sajjad, a spokesman for Punjab's rescue service, claimed the party wanted to the charter the boat despite warnings about the dangerously high waters.
"The family requested to cross the flooded river in the boat and were refused by military officials several times but they kept insisting," he said.
"The waves were moving fast and the family and other people panicked. They were asked to remain calm but they continued to panic, causing the boat to become unbalanced and capsize."
Floods and landslides from days of heavy monsoon rains have now claimed almost 500 lives in Pakistan and India.
Some 300 villages around Muzaffargarh have been inundated and the flooding has also devastated thousands of acres of the cotton crop.