Andrew Greening, of Linthorpe, Middlesbrough, was teaching his eight-year-old son the crawl at the Stockton-on-Tees pool when, in August 1994, he misjudged the distance to the water's edge, crashing face-first against the tiles, the court heard.
Mr Greening, who was 36 at the time, said the water in the irregularly shaped pool was 'so murky he could not see his feet clearly' and the edge of the water, which came up to the level of the surrounding walkway, was not marked clearly enough by a thin dark line.
Middlesbrough county court judge John Briggs ruled in March that Stockton-on-Tees BC, owners of the four-pool complex which has since closed, were 100 per cent to blame for Mr Greening's injuries.
'Given the murkiness of the water which Mr Greening had noticed and the difficulty which the county court judge found existed in seeing the edge of this pool, it seems to me he ought to have exercised greater care when approaching the edge,' said the judge.
Unless settlement terms are agreed in the meantime, Mr Greening's damages will have to be assessed at another court hearing. But the appeal court's ruling means his pay-out will be reduced by half.
The appeal court heard it was Mr Greening's third visit to the Stockton-on-Tees pool, which opened in 1984. He was a regular swimmer but normally visited a pool at Billingham.
'The accident happened at about 8am. Mr Greening with his brother and two sons aged eight and four were about the first people into the pool,' explained Lord Justice Simon Brown.
The 'leisure pool' with 'deck level water' had a uniform three-foot depth and was used mainly for play, he told the court.
There was 'no reason to interfere' with the county court judge's ruling that the council was at least partly responsible, but, explained Lord Justice Simon Brown: 'I think Mr Greening should have been more careful when approaching the edge, rather than assuming he had another stroke to go.
'I think he carries a very substantial share of the blame in this unusual and unfortunate accident. I find 50 per cent contributory negligence.'