A 21-year-old man has drowned in Bardney Lock near the Branston Fen on Thursday.
It happened at around 3pm on September 19, when he dived into the water and did not resurface, which was when emergency services were called.
Police officers, the Swift Rescue Water Team, the ambulance and an air ambulance attended the scene at around 3.15pm.
The body of a 21-year-old man was recovered from the water.
Sergeant Andrew Shepherd said: “Emergency services have sadly recovered the body of a man from the river, despite the best efforts of rescue.
“While work is now underway to understand how the man came into difficulty, I would like to take this opportunity to highlight this case as a reminder of the devastating consequences of entering open water – regardless of whether people do so deliberately or inadvertently.
“As in this tragic case, open water can have hidden dangers that can prove fatal and I would urge anyone who spends any time on or near open water to use this case as a devastating reminder of that – particularly during the recent warm weather we have been experiencing.”
The death is not being treated as suspicious and the matter has been passed to the coroner.