If a PC has a spell or item that allows them to breathe underwater, gives them gills, or whatever other means they might acquire to not drown, what is the maximum depth they could:

* Descend to before buoyancy prevents you from going any deeper?
* Descend before the pressure would become fatal?

Assuming we determine the maximum depth *possible*, how far could a character descend unassisted before their natural buoyancy prevents them from descending deeper?