“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 crimestoppers-uk.org
The police then later said: “We’re 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 us with enquiries.”