When Ishmael asks "Who ain't a slave?" he is asking a rhetorical questions because he means to cause an effect. Ishmael is trying to say that, in one way or another, we are all slaves to someone.
He continues to say "...I have the satisfaction of knowing that it is all right; that everybody else is one way or other served in much the same way..." In this part we clearly see that he is trying to say that we are all slaves, that we are all treated unfairly and with no mercy by a boss in our job, like he is mistreated by the sea captains.
Ishmael is taking a stand, he's affirming that this is a universal truth and it is true for everyone.