Officers were called to Swanwick Lakes at 8.21pm on Sunday, May 30. South Central Ambulance Service and Hampshire and Isle of Wight Fire and Rescue were also called, with firefighters taking two-year-old Greyson Birch out of the water.
Greyson, from Sholing, Southampton was taken to Southampton General Hospital where he died on Thursday, June 3.
An 18-year-old man from Southampton who knew Greyson was arrested on suspicion of neglect as part of the police’s investigation.
Now the force has decided to release him 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.’