To pursue or continue with some futile task or activity, wasting one's time as a result. Usually used in the continuous tense. I've spent my life campaigning for environmental protection, but with the way the world's governments continue to favor corporations over everything else, I feel as though I've been plowing water this whole time. You're plowing water if you think you're going to change Henry's mind.
n. wasting time doing something futile. You’re wasting your time. You’re plowing water.