Researching an area prior to a trip is always worthwhile. I was looking at the history of some wrecks. the map showed some visibible wrecks off Aberlady. Some further digging revealed that the wrecks in this case were minature submarines, dating back from World War II. These small craft were towed to enemy destinations by a 'mother' submarine. There were then set free with a crew of two, who navigated them to their target; their small size enabling to negotiate nets and defences. The potted history can be found here and here.
The craft at Aberlady were tied to a concrete block, and used for weapons testing by the RAF.
3 hours ago