Who are the "singers" in Walt Whitman's poem "I Hear America Singing"?

Whitman tells us exactly who the singers are.  He lists them out.  There are mechanics and masons, carpenters and boatmen.  There are men and women -- we see women sewing and washing.  All of them are singing and their songs are uniquely their own.

All of these people, in my opinion, are not really important for their specific professions.  In other words, it doesn't matter if Whitman has them being masons or fishermen.  What's important here is that they are all common Americans.  Whitman is saying that it is the common people who make America what it is.