Carry Someone's Water Meaning

(intransitive, idiomatic) To do someone's bidding; to serve someone's interests.

Example: 1996, James Carroll, The City Below‎, page 287:
  "I hear things are good." / Moran shrugged. "Depends on who you're talking to. Who would you be talking to, Squire?" / "Frank" / "I thought that might be it. You still carrying his water?
2005, Martin Clark, Plain Heathen Mischief‎, page 394:
  I've discovered the Lord doesn't need lackeys, lieutenants, minions, representatives and envoys to carry His water and discharge His affairs.
2006, James Bruno, Chasm‎, page 113:
  "Nope. Just here to see a staffer about something." / "Staffers. Yeah, well. They hold the Congressman's bag and carry his water sometimes all right."