In direct contradiction to the previous answers. Jack is not described or defined as a fishman according to the one piece wiki linked here Jack Description link
Jack is a man of massive stature, dwarfing his crewmates and even the very large minks Inuarashi and Nekomamushi.
Key phrase being Jack is a man. Given that there is also substantial evidence adding to the credibility that he is in fact human.
Fishmen in appearance
It is further described in the wiki that Fishmen are described as the following:
Fishmen are more fish-like than merfolk, usually looking like a combination between a man and a fish or other aquatic creature, such as an octopus, manta ray, or sawshark; however, they still have legs (in fact, Decken has four). They also have gills between their shoulders and necks, sometimes covered by their clothing, as well as often having webbed hands. Depending on species, they may have multiple limbs (mainly extra arms).
Given that all fishmen have some sort of gills between their shoulders and neck and that we have seen Jack full frontal topless and he's shown no signs of these due to the fur coat that he wears.
This can mean 1 of 3 things:
1) He is not a fishman and there is a logical possibility for why being above head level submerged in water wasn't causing him to drown, die or freak out
2) He is a merefolk but it hasn't been fully discussed yet and his lineage and capability.
3) He is actually a fishmen but couldn't move due to restrictions via devil fruit powers.
I disagree with reasons 2 and 3 because what sense would that make? He is the first antagonist in the series to not face any of the straw hats bringing him back either in the current whole cake arc or the next arc (hopefully wano country) ruins that nieche for his character. If he was in fact fishman or merefolk why didn't we see or hear anything about him from the fishman island arc? Or Jinbe? Or Kienmon and momonosuke? It doesnt make logical sense that all of those ppl just conveniently forgot him.
In summation I believe that reason 1 is the fact here. But that leads to the burning question, how is he ok being underwater?
There are 2 possibilities in this case, the first is that as a zoan type he could have matriculated his lungs to a point where they hold more air without expanding the rest of his body due to enhanced control over his zoan power.
The second and most likely case is that he is in fact drowning to death but hes such a tough bada** that he doesn't show that he even cares and might actually survive.
Truth of the matter is
Nobody actually knows for sure beccause it's too soon to tell, we will know for sure once the manga reaches a point where the straw hats or someone deals with Kaido and we learn a bit more about the beast pirates. But as of right now with the information we have in front of us. Jack is a man.