Swimmer's ear, also known as otitis externa, is an infection in the outer ear canal. It's often brought on by water that remains in your ear (swimming, bathing, etc.) that aids the growth of bacteria, fungus or even viruses. Usually you can treat swimmer's ear with eardrops.
Especially swimming in untreated water as lakes, ponds or the ocean makes one more likely to get irritation and or an infection of the external ear canal. Preventive measure (if you know your eardrum is intact), are to use OTC drying drops or home remedy drops equal parts of isopropyl alcohol and and white vinegar after swimming.
Swimmer's ear symptoms are usually mild at first, but they can worsen if your infection isn't treated or spreads. Contact your doctor if you have even mild signs or symptoms of swimmer's ear.