The Antikythera Mechanism, National Archaeological Museum of Athens
The next item on this list no longer exists underwater, but it deserves an honorable mention because it was such a historic and amazing find! Back in 1902, a group of fishermen stumbled upon this strange structure off the coast of the Greek island Antikythera. After they reported their discovery, excavators went over the site and determined this structure is the oldest known computer in the world. It’s estimated to have been invented sometime 200 and 70 BC. Once they removed the Antikythera Mechanism, it was moved to the National Archaeological Museum of Athens.