The 500 pound World War Two device was picked up during dredging work.
Double Silver Rounds Off The Sailing
Britain have finished the sailing events in Weymouth and Portland with two silver medals.
Weymouth's Luke Patience and Portland's Stuart Bithell had to settle for silver behind Australia and were full of praise for the new Olympic champions.
''We tried it, we got the better of them at the start and just managed to just sneak ahead,'' Bithell said.
''Unfortunately we sneaked ahead of the rest of the fleet as well. We had hoped they would be there as well.
''They sailed a fantastic race, we tried to attack them when we could and hats off to them. They held their nerve and are worthy champions. We're happy with second.''
Despite missing out on gold, Patience and Bithell were confident they would be back to the 2016 Olympics in Rio.
But there was no regret from the Britons after crossing the finish line at Weymouth behind Australia.
Patience said: ''It's not gold but there's plenty of time. We're young boys and we'll be coming back and we'll mean business next time round yet again.''
But Hannah Mills and Weymouth's Saskia Clark were upset to miss out on winning another British gold.
The duo, together for just 18 months, were beaten in the final by New Zealand.
Clark said: ''I'm gutted at the moment. We didn't even put a good show on really.''
An emotional Mills said: ''I'm pretty gutted to be honest. But we've had a wicked 18 months together and Sas is amazing, we've had a great time.''
Elsewhere there was more British success when Lutalo Muhammad, a controversial selection for the taekwondo team ahead of world number one, Dorset's Aaron Cook, won his first-round contest to advance to the quarter-finals.
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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