If whoever build this thing did so in a place that was under water at high tide, wouldn't the wood have rotted away before being preserved in peat?
I just remembered something I read about England some time back. I saw a map that showed where the Ice Age glaciers covered only half of England and the northern part had subsided under the weight of the ice and that even to this day the northern part was still rising and the southern part was sinking. This would explain why it is under water. It was originally built on high dry ground and then sank?