Edinburgh Sheriff Court was told Andrew Lindley, 37, left Tamoor Tariq with a 1in stab wound to his lower abdomen and a cut to the left of his face.
Fiscal depute Ruth Ross-Davie said: "It would appear Mr Tariq and others were mucking about with water balloons and, as a consequence, the girlfriend of Lindley was splashed.
"A few minutes after that Lindley charged into the shop producing a knife and striking Mr Tariq repeatedly to the head and body with it.
"This incident was wholly captured on CCTV.
"Numerous calls to the police were made in relation to the incident and in due course Lindley was identified as the culprit."
The court heard the injury to Mr Tariq's abdomen required an operation under general anaesthetic. His face is also scarred.
Lindley admitted the attack at Dundee Street, Edinburgh, on 15 June last year.
Sheriff Derrick McIntyre deferred sentence on Lindley, of Northcote Street, Edinburgh, for background reports until next month, when pleas in mitigation will also be heard.