The vicious cycle of yeast, bacteria, and infection
The reason why you want to clean and dry your dog’s ears after swimming or bathing is because the water that gets into their ears during these activities is likely to create a warm, moist environment within their ears that will allow for an overgrowth of the yeast and/or bacteria that are normally present on their skin and in their ears. This increased growth of yeast and/or bacteria will irritate and inflame your dog’s ears, starting a vicious cycle that will make the infection and pain even worse.
Make sure you're using the right ear cleaner
You don't want to just grab any old ear cleaner, and you certainly don’t want to reach for the bottle of rubbing alcohol! Some ear cleaners aren't very effective, and some don't help to dry your dog's ears. Ingredients like rubbing alcohol and witch hazel, though they can help the drying process, can be extremely irritating and painful, especially if your dog's ears are already red or cracked.
There are several good ear cleaners/dryers out there for such routine cleaning. Here are a couple that you can easily pick up online, at your local pet store, or even at your vet's office.
If you’re getting a lot of discolored discharge from your dog’s ears when cleaning them, it’s time for a trip to your vet. Often times this indicates that an infection is already starting to take hold and getting your vet involved sooner rather than later is definitely in your dog’s best interest, and it’s in yours too! Ear infections get a lot more painful, and a lot more expensive to treat, the longer they’re left to linger.
Prevention is the best medicine
You can help your dog (and yourself) by never dumping a cup or pitcher of water over their head to rinse and never spraying their head with the shower nozzle or an outdoor hose. Regardless of how careful you think you might be, it’s a sure-fire way to get water in their ears! Decrease the risk by washing and rinsing their head with just a damp washcloth or sponge. Of course, it's tough to prevent water from getting in your dog's ears when they're swimming... it's not like they're going to be excited about wearing a swim cap! So do what you can, when you can, to prevent water from getting into your dog's ears, and then always clean and dry them after bathing or swimming.