The man, who is in his 30s but has not yet been named, was pulled from the water by members of the public and taken by ambulance to hospital.
Despite the best efforts of medical staff was pronounced dead a short time later.
Detectives now want to hear from a man in a black hat who said he had witnessed the deceased enter the water.
T/DI Ellie Gomerson, of South Division, said: “First and foremost my thoughts are with the deceased man’s loved ones at this incredibly difficult time.
“The man appears to have died in tragic circumstances and his death is not being treated as suspicious.
“As part of our ongoing investigation, we do need to speak to a witness who said he saw the deceased going into the water. I would ask that individual to please contact the police as soon as possible.”
Anyone with information should contact police on 101 quoting log 0306 of January 24.