Patrick Cahill, 73, showered a man having a meal with his family after getting "fed up" with noise coming from the La Bodega tapas bar in swanky Notting Hill, west London.
Cahill admitted a charge of common assault in connection with the incident on the evening of Friday July 12 when he appeared at Westminster Magistrates' Court.
The pensioner claimed he hadn't slept for three months because of the noise.
Misha Majid, prosecuting, said: "At around 9.50pm in the evening the victim in the matter was at La Bodega restaurant on Tavistock Road.
"There are residential properties above that premises.
"The victim was present there having dinner with his family when all of a sudden he felt liquid pour down on him.
"When he looked up he saw that the defendant had thrown a bucket of water from the flat above."
She said police were satisfied the liquid was water, although the victim thought he could smell bleach.
Ms Majid added: "Given that water was used, although not a weapon, the Crown would say it was a particularly nasty offence.
"I do believe in his own statement he has set out he was surprised and shocked by the incident."
She said Cahill, who had five previous convictions, was arrested, adding: "During interview he explained that a lot of noise rises from the restaurant and he became fed up and, as a result, he had thrown water from his property, resulting in it falling below."
Cassan Lindsay, in mitigation, said: "What Mr Cahill explained to me is that he lived in accommodation for elderly people and has been there for three months.
"He went to view the flat, he said it was a nice flat when he went there. It was double glazed windows so he couldn't hear anything and he decided to accept the accommodation.
"He says since then he hasn't slept properly for the past three months. The difficulty is that the flat gets really warm. To sleep, what he has to do is open the window."
Mr Lindsay added: "He accepts his behaviour was of course inappropriate and it was something he should not have done, but he has been very stressed by the situation.
"He said to me he just wants one night's sleep. One can imagine over the past week just how hot it has been."
Chairman of the bench Terry Carpenter adjourned the hearing on Monday for pre-sentence reporting to be prepared.