Poorly maintained poolside areas and changing rooms can lead to warts on the sole of the foot. They are rarely dangerous in themselves, but the pressure of walking can drive the wart into the skin. It will stay embedded until removed by a doctor. That doesn't hurt too much, but getting around afterwards certainly does. So avoid swimming if you have any sort of cut, however superficial.
Rampant impetigo can infect the bloodstream and lead to kidney disease.
Germs also find their way into the smallest of scratches caused by depilation, including shaving or waxing. Caused by suntan oil, perspiration and algae, the dirt ring gets worse as the number of swimmers increases. Many public pools use a session system, asking bathers to get out every few hours so that the contamination level can be checked. But that may still not be enough. Still, swimming should be fun, and not a prelude to a visit to the doctor.