The 500 pound World War Two device was picked up during dredging work.
Arrest In Murder Inquiry In Bournemouth
Detectives are continuing to question a man in connection the murder of a 21-year-old who was shot dead in a 'pre-planned' attack.
The suspect, 22, was arrested following the killing in Bournemouth during the early hours of Wednesday morning.
Dorset Police said they were called to an address in the Boscombe area of the coastal town at 2.26am to reports of a seriously injured man.
The victim, believed to be a 21-year-old man from London who is yet to be formally identified, was declared dead at the scene after sustaining a gunshot wound.
Police have renewed their appeals for witnesses and anyone with information to come forward.
Detective Chief Inspector Kevin Connolly, of Dorset Police's Major Crime Team, said: "We are still investigating the circumstances of this death but I believe this was a pre-planned and targeted attack by a number of offenders.
"We are following various lines of enquiries in an extensive investigation and have arrested a 22-year-old man in connection with this incident who is helping us with our enquiries.
"I would like to reassure the local community that there is no immediate concern for the safety of the public."
Witnesses and anyone with information should call Dorset Police quoting incident number 25:46 or Crimestoppers anonymously on 0800 555 111.
640 men were killed when the South African troop carrier was hit by a cargo ship in 1917.
The 79-year-old man suffered a serious head injury in the Cranmer Road car park in Winton.
Two Girls Indecently Assaulted Last Sunday
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