Birdham Pool is, I believe, 19th century. Chi Yacht Basin is post war. Leading up to 'D' day the area now used by Chi marina was used for storing lighters which were filled with necessary supplies. After the war the remaining lighters were sold off as a job lot, with the condition that they were removed prior to the marina being built. The story goes that the chap who bid for them made a fortune as two of the lighters were filled with bronze fastenings which he sold on and he then removed the rest of the lighters.
This is hearsay from having worked in the marina for a good few years back in the sixties when the yacht basin had only just opened. I believe it.
My guess is that the hulk is the remains of one of the lighters from this war-time period.
It has the right shape !
An aside - when I was working there (50 years ago !) a chap hired me at a very specific time and date to take him out to a mudbank off Thorney Island were there was a sunken aircraft. Low tide springs, he jumped onto the wreck with a jewellers file and an envelope, spent some time and the came back aboard the dinghy - when we got back to land he gave me a fiver ( third of a weeks wages for me , then) . What was that ? - he explained that he had filed the platinum off various parts of the aircraft's display and that he was going to get £ 100 for it .......