That's an 18" DOUBLE MOORING BITT he is sitting on.
USS Kawishiwi AO-146   1957

The black, bag looking item is Bob's Hammock. The nights got pretty hot down below.
I remember this location, vividly. While mooring in Philadelphia after the 1st sea trial, I nearly got my foot caught in the line as it was being paid out.
 

Photos by: Bob Lewandowski.....SF 55-57crew