Mr Martin, a founder member of advertising giants Saatchi and Saatchi, had drowned in a few inches of water in the tank beneath the grounds of his family's cottage, police said.
But officers investigating the death in the village of Steep, near Petersfield, Hampshire, declined to comment on why he was in the darkened tank, believed to be used for collecting rainwater. "Nothing further is being said at this stage," said a Hampshire police spokesman. "A report is being prepared for the coroner. It is an unusual incident."
After Mr Martin's body was found by his brother, who called at his home on Thursday night, police said the death was being treated as suspicious.
According to one report, not confirmed by police, the area had been searched by officers earlier after Mr Martin's cries for help were heard three- quarters of a mile away.
Mr Martin, whose wife and children were away from home, was believed to have been lying dead in the underground chamber for no more than a few hours when he was found. Detective Superintendent Andy Longman, leading the inquiry, said the death seemed to be an "unfortunate accident". It was clear Mr Martin had drowned, he said. "However, we still have to make various inquiries. I cannot say we are totally happy about the circumstances of his death. It appears to be a very strange incident. I would not think it was a suicide."
Mr Martin was involved at the launch of Saatchi and Saatchi in 1970 and many in the industry believed his work helped the agency to achieve its rapid success. He left in 1985. One said he was "the greatest copywriter in Britain".
He left Saatchi in 1985 - before the final expansion of the company and the dramatic changes which followed the departure of the Saatchi brothers last year - and jointly set up another advertising agency, Edwards, Martin and Thornton.Reuse content