Swimmer's ear is a painful infection of the skin lining the outer ear canal. There are non-effective bacteria that normally lodge on the surface of this skin. But once the skin breaks, bacteria can get inside and cause an infection called swimmer' ear (externa otitis). Swimmer's ear often attacks swimmers due to their increased exposure to water.
If your ears are wet for quite a while, the skin lining become softer and wrinkled. Any slight scratch like sticking a finger in the ear canal may open the way for an infection. When the skin is much softer due to wetness, bacteria can easily dig in and cause an infection. It is ironic then that very dry skin may crack, also causing a swimmer's ear.
Swimmer's ear is more common in people who swim in chlorinated water pools than in divers or in people who swim in the sea. This might be due to the effectiveness of chlorine in pool water to kill good bacteria that lodges in the ear. Chlorinated water is good, but it is not as effective in killing harmful bacteria that cause an infection, rather it softens your skin leaving it vulnerable to infection. Therefore, it is advisable that you reduce the amount of time you spend in pools, if needs be.
Most cases of otitis externa are reported to be in summer when swimming is more common. For unknown reasons, swimmer's ear is more prevalent in school children and adults than in infants.
The above information thankfully comes from the entmags.org at the following link.