What's a scow?

A scow is a type of large boat with a flat bottom and squared ends that's used to transport material (sand, trash, stones, etc.) in bulk. Tugboats often tow this type of boat.

I went one day down on the wharf of Mr. Waters; and seeing two Irishmen unloading a scow of stone, I went, unasked, and helped them. When we had finished, one of them came to me and asked me if I were a slave. I told him I was. He asked, "Are ye a slave for life?" I told him that I was.

From The Narrative of the Life of Frederick Douglass: An American Slave