Scientists found that even on beaches where the seawater quality meets the European Union standards, bathers run the risk of catching infections.
Studies in Santander, northern Spain, showed some swimmers and beach users were suffering gastrointestinal, skin and repiratory problems.
A recent guide by the Marine Conservation Society (MCS) on the quality of UK coastal waters said there has been "a massive improvement".
But this latest research, published in the Journal of Epidemiology and Community Health, highlights worries that seawater quality standards are still not high enough.
Cleaner beaches
Vicky Garner, campaign manager for Surfers Against Sewage, said they had campaigned for over a decade to get cleaner beaches.
She said UK surfers using "safe" British beaches were still suffering a variety of illnesses after going in the water.
"We are getting sick and the beaches are covered in sewage.
"This research does not surprise us at all and this is why we are calling for a revision of the bathing water directive," she said.
She said one of the problems is that beaches are given their ratings on testing results from the previous summer and that during this time they might have deteriorated.
Bacterial worries
The Spanish study revealed that 7.5% of the 2000 people quizzed on the Spanish beaches between July and September 1998 suffered problems.
Visitors were found to suffer more health problems than locals and their symptoms correlated with bacterial levels.
Levels of bacteria were found to be between 2,5000 and 10,000 for every 100ml of seaweed.
This made swimmers 10 times as vulnerable as non-swimmers - a level above the European Union guide standard for microbial contamination.
Almost 40% of water samples were found to exceed the guide standards for bacteria.
Subjective reports
But Dr Gordon Nichols, a PHLS scientist, said there was always a problem using self-reported symptoms, as people could exaggerate the seriousness of their illness.
"The difficulty with self-reported symptoms is that they are by their very nature dependent on the accuracy of the person reporting them.
"The intensity of the symptoms is subjective so it is difficult to know whether the symptoms are mild or severe."
Dr Nichols agreed that there was currently a move to get a different classification of the beach pollution.