The emergency services were called to Pitsford Reservoir after a yachtsman fell into the water.
Northamptonshire Fire and Rescue Service received a report that a person had fallen from a yacht into the reservoir just before 3.20pm yesterday, Friday, May 10.
Fire engines from Moulton and The Mounts and water rescue vehicles from Mereway and Wellingborough attended the scene,
The man, who is in his 60s, was rescued before they arrived.
Warwickshire & Northamptonshire Air Ambulance also attended and the on-board doctor treated the man at the scene.
He was suffering from hypothermia as he had been in the water for more than half an hour.
He was transferred from the air ambulance to a land ambulance at the scene before being taken to hospital.