Concrete barge off the Port of Bangor, always submerged at high tide.
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There is one of these on the Orwell river at Ipswich in Suffolk UK. It was derelict a couple of years ago but last summer it was being renovated (is that what you do to a Barge?) We have a plan to go down this spring and see what progress has been made. These were transport barges built for the D-Day invasion and even those that are submerged seem to have lasted very well. Just wish good luck to the owners, really historic vessel.