Officers were called to the reserve at 8.21pm on Sunday 30 May, and attended along with the ambulance service and fire service, who got two-year-old Greyson Birch out of the water.
Greyson was subsequently taken to Southampton General Hospital where he sadly died on Thursday 3 June.
Hampshire Constabulary has conducted a lengthy and thorough investigation into the circumstances surrounding Greyson’s entry to the water, and his subsequent death.
As part of enquiries, an 18-year-old man from Southampton, who was known to Greyson, was arrested on suspicion of neglect.
On the basis of all evidence available to police, a decision has been made to release the man with no further action.
Detective Inspector Matt Gillooly said: “Greyson’s death was a tragedy, and our thoughts remain with his family and everyone who knew him.
“Officers have conducted a thorough investigation into this tragic incident, and we want to thank everyone in the community who responded to our appeals and assisted investigators.
“The criminal investigation has now concluded, and police will be working alongside the Coroner’s office in a continued effort to examine the circumstances surrounding Greyson’s death.”