The seal between the corwn and the case.
If you have to wind it every 24 hours, you damage the seal faster than with an auto movement or a 8 Days power reserve.
Yes, you're right, the first Rolex Panerai watches ( 2533 / 3646 ) housed a Role Cortebert manual movement with a shot power reserve, but there were the first Diving watches, and after using the same movement on the 6152, Panerai opted for an 8 Days Angelus movement.
Since the birth of which is considered as the roots of the Diving watches ( Blancpain and Rolex ), you will notice that all the diving watches come with automatic movements.
Do you know one short power rserve manual winding movement housed in a diving watch?
I don't.
Best, my friend.
Nicolas