'Its six o'clock, in the hold below
Sundown sundown below,
time for us to roll and go'

This is a stevedores song and marks the end of the days work, the ship is not yet ready to sail as a later verse makes clear. This is in line with Hugill's explanation, but of course, a with all real language issues, there may have been more than one origin, and later singers were free to incorporate it if it fitted the rhythm.