How was Jack alive underwater?
He is of the Fishman race. Eating the devil fruit made him still but his fishman nature helped him breathe underwater. The devil fruit wont remove that ability. He survived underwater mainly because he is of the fishman species.
Is Jack from One Piece still alive?
However,Jack is still alive. He appeares to be half fishman,just like Dellinger from Donquixote family,so he is able to breath under water,but he can’t move because he is a devil fruit user.