Diving bell

The diving bell was a chamber lowered into the sea to allow divers to explore underwater for prolonged periods of time, as they can return to the bell which retained air from the surface for them to breathe. Pirate captain and Assassin Edward Kenway used a diving bell during his time in the West Indies to explore various shipwrecks and salvage treasure, or, in the case of the sunken Spanish Treasure Fleet, medicines.

Reference

 * Assassin's Creed IV: Black Flag