The 500 pound World War Two device was picked up during dredging work.
Dorset PCC Becomes Crime Victim
Dorset's first police and crime commissioner has become the victim of theft while at a meeting discussing crime in the community.
Martyn Underhill's force pool car was broken into and the sat-nav stolen while the vehicle was parked at the Sovereign Centre in Boscombe, Bournemouth, where he was attending a meeting on Monday.
Mr Underhill said: "We all know Boscombe does have a crime issue. The irony of this incident has not been lost on me.''
He had spent just over two hours at the meeting, discussing plans to create an unpaid advocate to support the commissioner in helping to address community issues.
Mr Underhill said: "I accidentally left my driver's window open by one inch, someone forced the window down and grabbed the sat-nav from the windscreen.
"The charger unit was connected, and snapped off in the socket in the centre console, as they pulled the sat-nav out. No actual entry to the car was gained, it was left locked.''
He added: "That is why I am employing a voluntary advocate, to help me to work with partners to resolve the issues in Boscombe. This incident also raises awareness of vehicle crime in Dorset.
"We are working on reducing vehicle crime and it is one of the force's priorities.''
The independent candidate put keeping politics out of policing at the heart of his campaign to be elected last November, also pledging to keep officers on the streets and the supervision of offenders to reduce re-offending.
Mr Underhill, who helped lead the investigation into the disappearance of missing schoolgirl Sarah Payne in 2000, beat Conservative candidate and businessman Nick King to the role.
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.
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