It's from the South Pacific Ocean, around the same area of the real-life Bikini Atoll. It's a pretty big rumour that BB is a nuclear result of atomic bombings in the '40s by the U.S, which realistically also affected Bikini Atoll.
i highly doubt that rumour is true, its a cartoon, they can do whatever they want.
Obviously, it is definitely not the Antarctic or Arctic oceans, unless one has fewer than a single brain cell.
It's sensibly true.
Bikini ATOLL
Bikini BOTTOM.