Nicolas Robinson, 23, of Borough, south-east London, carried out the “opportunistic” theft at a Lidl supermarket in Brixton as he walked home from his girlfriend’s house.
Robinson threw away the water and ran when he was confronted by police but was arrested and quickly admitted what he had done.
His solicitor told Camberwell Magistrates’ Court had “got caught up in the moment” and was “incredibly ashamed”.
But District Judge Alan Baldwin said the background of “serious public disorder” was an aggravating feature.
Members of Robinson's family in the public gallery gasped with disbelief as the judge told him he would be going to prison.
I bet you're over the moon. Six months, you sick bastard, six months. Fuck you, vigilante scum. You have knifed a whole generation in the back, again and again, now you want them to limp off to jail to make you feel a little safer in debtlogged little suburban castle where you hoard your meagre treasures from a lifetime of petty grubbing and forelock tugging. Fuck you. You are the guilty one. You have stood by while your children's future is ransacked. Fuck you. When this madness is done they have every right to tear your little world apart.