Most of the 23 passengers and crew on board reached the banks of the scenic Tenryu river, a local fire department official said.
Four of them had to be taken to hospital with one, a woman in her 70s, unconscious, and she later died.
The 12-metre (40-feet) long fibreglass boat, overturned in mid-afternoon on the Tenryu, which flows out to the Pacific some 200 kilometres (125 miles) west of Tokyo and attracts crowds of visitors in summer.
"We usually give a life vest to each passenger, but we cannot confirm yet if they really put on the vests," a spokeswoman for the boat operator Tenryu Hamanako Railway said. Source