Hello, I am fairly sure it came from a ship. See how the end is clenched over? When I built my boat, the old Boatbuilder that was teaching me would drive the nail (turbine bronze) from one side, while I held a "bucking iron" on the other side as the nail came through. The bucking iron would cause the nail to bend over and as the Boatbuilder keeps driving the nail, it draws the two boards tighter and tighter. Once the head of the nail is flush with one side, and the tip, is clenched over and flush on the other, it will continue to hold until the wood fails years and years later. Looks like your nail was in place long after the ship sank, and the wood rotted away. If it had been pulled, it would have uncurled. I hope this helps. George.