According to New Hampshire Fish and Game Sgt. Dave Walsh, Thomas Howell, 57, Hinsdale, was not wearing a life vest when he paddled out onto the river just to enjoy the day.
Howell had been dropped off by his girlfriend on Thursday morning at 9:30 at the Prospect Street Boat Ramp in Hinsdale.
The woman returned at 12:30 to pick him up and saw him swimming alongside the kayak in the middle of the river, said Walsh.
"She took her eyes off him and when she looked up he was completely gone," he said.
A search was launched shortly after she called for emergency services.
Searchers remained on the scene until about 6 p.m. on Thursday night and returned at 8 a.m. today.
Howell's body was found about 40 yards from his last observed location, in about 15 feet of water, said Walsh.
The medical examiner ruled it a death by drowning, said Walsh.
Walsh said it appeared alcohol was a factor in Howell's death.
"There was some alcohol found in the kayak," he said.
New Hamphsire Fish and Game was assisted by the Brattleboro Fire Department, N.H. Marine Patrol, the Hinsdale Police and Fire departments and the Northfield, Mass., Fire Department.