Remember? Spongebob was like "air? Oh I love air." in an attempt to impress her even though he didn't actually know what it was. So he lead her to believe that.
she may have also been smart enough to know that a sponge is not a sea creature and shouldn't breathe at all.
Obviously, she was smart enough to work around that problem.
I appreciate your avatar.
She WAS pretty new to the ocean world at that point... ?