Officers were called at 8.21pm on Sunday 31st May 2021 they attended along with the ambulance service and fire service, who got the two-year-old boy out of the water.
The child was subsequently taken to Southampton General Hospital where he remains in a critical but stable condition.
Police are investigating the circumstances surrounding how the boy came to be in the water, and officers would like to hear from anyone who was in this area between 7.45pm and 8.30pm.
An 18-year-old man from Southampton, who is known to the boy, has been arrested in connection with this incident and is assisting officers with enquiries.
Detective Inspector Toby Elcock said: “We know this incident will come as a shock to the local community, but we ask people to please not speculate as we follow up all lines of enquiry to establish how the boy came to be in the water.
“We are keeping an open mind as to what has happened, and we are keen to speak to anyone in the area to help us build up a picture of the circumstances.
“If you were in the area at this time, please call us. Any information, no matter how small, could prove significant, so please do get in touch.”
Anyone with information is asked to contact police on 101, quoting 44210210811.
Alternatively, you can contact the independent charity Crimestoppers 100% anonymously on 0800 555 111, or via their anonymous online form at http://orlo.uk/LfFUt