After a bit of background research, I’ve found out that diving for shipwrecks did happen, such as this case, for example:
By the time Jennings arrived in Florida in early 1716, most of the treasure from the 1715 Treasure Fleet had already been returned to Havana after collection by Indian divers. (Henry Jennings, Wikipedia)
But I’m wondering, how would the process go about for divers to actually retrieve the treasure?
Would a rowboat be sent out on the water above the wreck, in which the diver jumps off and dives to search for the treasure? Or would they swim out from the shore to the wreck?
How would the diver retrieve the treasure? By swimming with his hands full of it, or putting it in a bag, and bringing it up to the rowboat?
After the Nuestra Señora de Atocha shipwreck of 1622, the Spanish sent Natives to retrieve treasure. They had dropped grappling hooks into the water and waited until it snagged something. The Natives were then sent down holding large rocks. When they retrieved the treasure, they would have dropped the rocks to come back up.