I think his pointy teeth could also very likely be an indicator that he's a fishman, coupled with the fact that, as you mentioned, he's still conscious at the bottom of the sea and not drowning.This is the page I think you're both talking about (ch824).
We don't know that he is a fishman, but unlikely that he is, otherwise we would've seen something from his appearance or Oda would've mentioned it in his infobox.
The biggest theory here would be how is Jack able to remain conscious for so long underwater? Having been knocked to sea the previous day...
I mean, that makes more sense rather than "Jack is so OP that he can hold his breath for hours/days/weeks".