A man who was rescued from the water at Tring’s Cowroast Marina could not be saved by paramedics.
The emergency services rushed to the mooring spot just after 3.30pm on Thursday after a man, aged in his 40s, was spotted in the water.
Reports suggest he may have been trapped under a boat.
Tring firefighters pulled him from the marina and the ambulance service was called.
A spokesman for the East of England Ambulance Service said: “An ambulance, rapid response vehicle and an ambulance officer attended.
“Unfortunately nothing could be done for the patient and the man died at the scene.”
Herts Police said the death is not being treated as suspicious and the case has been passed on to the coroner for Hertfordshire.
The marina, at the summit of the Grand Union Canal, offers mooring for boaters and the man had been keeping his boat there for around four months.
The site is run by the British Waterways Marinas Ltd, which operates 20 marinas throughout the country, including Apsley Marina in Hemel Hempstead.