The flood swept through a canyon on the Verde River inside Tonto National Forest, about 145 kilometers northeast of Phoenix, late Saturday.
There was no warning before a wall of water hit the swimmers.
'They had no warning. They heard a roar, and it was on top of them,'' said Ron Sattelmaier, chief of the fire and medical district around Water Wheel Falls, Arizona.
Rescuers recovered three bodies Saturday and six more on Sunday, when they also said a 13-year-old boy was still missing.
Four adults and five children drowned. The Arizona Republic said officials on the scene believed all were members of the same extended family.
Up to 4 centimeters of rain from an intense thunderstorm had fallen during one hour in a remote area about 14 kilometers upstream, local authorities said. The storm surge of water, swollen by debris from recent wildfires, reached a narrow canyon where people were taking refuge from the summer heat. The river burst its banks and overwhelmed the swimmers.
One of the first responders told a reporter a black wall of water almost two meters high and more than 10 meters wide rushed through the narrow canyon at nearly 80 kilometers per hour.
A hundred or more people people may have been relaxing in the water at the time, but authorities said late Sunday they were confident everyone had been accounted for, apart from the missing teenager. Witnesses said survivors clung to trees as the water rose around them.
The severe thunderstorm that caused the sudden flood pounded down on an area charred by a recent wildfire, which left a deep burn scar on ground where pine trees, foliage and ground dust would normally absorb rain.
'If it's an intense burn, it creates a glaze on the surface that just repels water,'' said Darren McCollum, a meteorologist.
The National Weather Service had issued a specific flash-flood warning about an hour and a half before the tragedy in Tonto National Forest unfolded.
Most of Arizona is under flash flood watch until Monday evening, and the regional NWS center in Phoenix warned that 'heavy rains can produce flash flooding of low lying, normally dry areas such as washes.'