According to the recent Vivre Card Databook – Vol.11, Jack the Drought is confirmed to be a full fishman (not half human), being a brindlebass fishman.
Because of this, even when falling into the water, while not being able to move due to his devil fruit, Jack is still able to breathe.
It is also confirmed his devil fruit being the Zou Zou no Mi, Model: Mammoth, an Ancient Zoan-type Devil Fruit, which allows the user to transform into a mammoth hybrid and full mammoth at will.
The full mammoth form granted by this fruit gives the user a massive frame, significantly increasing Jack’s already abnormal size. Jack can level buildings with a single swing of his trunk and leave imprints of his feet on the ground just by stepping on it.
It is also revealed that Jack is 28 years old. At some point he joined the Beasts Pirates and a latter point, he was eventually promoted to All-Star.
This seems to imply he didn’t found the crew with Kaido or something among those lines, though he was present 20 years ago at Wano when he was only 8 years old oddly enough.