Great question — answering as the UphillTech team:
Yes, Jack being a fishman is the only logical explanation for him surviving in the middle of the ocean. As you noted already, he looks like a fishman too. His devil fruit prevents him from moving but that won't stop him from breathing (remember in Arlong park, Luffy could breathe while his body was under water when Nojiko (Nami's sister) and Genzo got his head outside of the pool). It also seems it was not the first time he found himself at the bottom of the sea as he is expecting help as if it was normal, and we can assume he either can speak with nearby fishes or he has a vivre card on him, and is only going to die if nobody comes to his rescue before he dies from hunger.
Full answer by Shautieh: https://anime.stackexchange.com/a/34296 (CC BY-SA 4.0).
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