Last summer 14 people died by drowning in public or private pools and nearly 1,000 accidents happened in local authority pools alone.
Which? sent two expert pool inspectors to rate 29 pools across the country against major safety standards set down in the current guidelines.
All public and private pools are legally required to carry out a risk assessment to set safety standards based on the pool's size and features and the type of people using it.
The inspectors found that all 29 pools failed to meet some of the guidelines and some gave "serious cause for concern".
In the 27 pools where lifeguards were provided Which? says that only four met the published standards fully. More than two thirds of pools did not have adequate lifeguard cover - meaning there were not enough of them, not all areas of the pool were supervised or the lifeguards were not behaving professionally.
In the worst cases inspectors described lifeguards as "lacking in vigilance", "unprofessional" or "complacent". "This is particularly worrying as our inspectors could see clearly signed bather-operated alarms in only six of the 29 pools they visited," said the report.
Signs - essential to tell people where it is safe to dive or of sudden changes in pool depth - were often inconsistent and not sited in the best places. Often, some areas had no signs and several pools did not make it clear where only shallow diving was safe.
In most pools where there were any potentially dangerous features the management had taken measures to prevent problems occurring. However, several pools had worrying features including water-slide supports which could cause head injuries or powerful suction in water outlets in which hair could become trapped.
The CA says that at Bournemouth International Centre virtually the whole pool was supervised by a single lifeguard, and adults and children were swimming across designated swimming lanes.
At the Nova Centre in Prestatyn, north Wales, the inspectors described an unsupervised learner pool and said they felt the lifeguards showed an "general lack of professional behaviour and a high degree of complacency" with children running round the pool.
In the Sandcastle leisure pool in Blackpool support structures for two of the slides overhung the pool creating the potential for head injuries and two children banged their heads during the inspection. At the Rhyl Sun Centre in north Wales, the bottom of a slide overhung the pool close to the water surface with children diving under the water to explore the air pocket which could have led to an accident.
Bournemouth said it was "investigating" the alleged problem and Sandcastle said that it had taken steps over the slides although it thought the Which? criticism "very harsh".
Sue Appleton of Denbighshire County Council which runs both the Rhyl and Nova Centres said that in the case of Rhyl, the criticisms had been taken on board. But in the case of Nova Ms Appleton said that the council was "questioning the inspectors' judgement".
"They said Nova was a leisure pool when it's not. It's an ordinary tank pool. And on the day they visited there were seven lifeguards on duty when the statutory requirement is only three."
Editor of Which?, Helen Parker, said it was encouraged by the positive way that pool managers responded.Reuse content