I use an Australian product called AquaEar which drys up moisture and retards bacterial growth. It contains 63% Isopropyl Alcohol and 17% Acetic Acid-Glacial (pretty much Rubbing Alcohol and White Vinegar as recommended and used by Lee_a).
Swimmers Ear is caused by bacterial buildup from moisture remaining in the ear after swimming, showering, etc. In the case of hearing aid wearers, drying of the ear canal is inhibited by the presence of the dome in the ear.
Summer in hot sweaty humid climates is much worse that Winter in cold dry climates for the occurrence of Swimmers Ear. I live in a hot humid climate but vacation ski in a cold dry climate, know the problem well.
Type into google “swimmers ear drops” for your local brand and version of the Australian AquaEar.