Great question — answering as the UphillTech team:
We don't know. So, first of all, Jack is either an enhanced human or a species that breathes underwater. I don't think any indication (aside from sharp teeth) has ever been given implying merman. From the wiki : Jack is a man of massive stature but it can be argued that 'man' just refers to 'male' in this case. Anyway, up until now (Chapter 861), no indication regarding his species has been given. As to why he remains conscious, once more, there is no real explanation. From the wiki : Despite being a Devil Fruit user, Jack can remain conscious underwater for an extended period of time without any visible signs of strain, aside from his inability to move; how he achieves this is not...
Full answer by BlueMoon93: https://movies.stackexchange.com/a/71376 (CC BY-SA 4.0).
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