Possible Clues:

• Bathtub dirt
• Bathtub woe
• Impurities on top of water or worthless people
• The lowest film, but it's found on top
• The South is bad on top with the rest of it
• Worthless man in dirty film
• Dregs (of society?)
• Film's gross - round about
• Impurities on liquid's surface
• Impurities on water surface