Similar story I remember from the GC, probably in the 70s.
This particular day was a very low tide, & there's a bloke working on one of the trawlers, moored near where SeaWorld is now.
He has a look over the side & spots a long, skinny shape on the bottom. Nobody can work out what it is, so one of them dives in & finds that it's a 500lb (250kg) bomb!
Called the
, who called Air Force bomb disposal, who came down, lifted it into a rubber raft, towed it out to the middle of the Broadwater, sank it again & blew it. Water spout a couple of 00' high!
Best guess is that a trawler had picked it up out off Straddie, brought it back tangled in their net, eventually got it free, then just dropped it over the side!
Anybody's guess as to how long it had been sitting there though.