Oceans - 28 free documentaries about shipwrecks & sunken treasure.
Start with 1623 Lost Galleon: The Hunt For The World's Most Valuable Shipwreck (51 min, Timeline), then Ghost Ships of the Great Lakes: Lost Beneath the Waves (44 min, National Geographic). All 28 documentaries here are free to watch from their makers' official uploads. To read deeper, the top-rated book on shipwrecks & sunken treasure is Shadow Divers by Robert Kurson.