I'm not sure, but I think "swimmer's ear" can be caused by yeast. Have you tried a vinegar and water rinse? Vinegar makes the ph unfriendly to yeast, but if it the infection is well established, you may need a trip to the vet for some anti fungal meds. We keep a bottle of vinegar/water around all the time for after swim, just like they used to do at summer camp when we swam in the river!
If there's a lot of brown gunk and it smells funny, try cleaning it very gently with a cotton ball (or two, or three) after you put the vinegar/water solution in and your dood has run around shaking his head. If it doesn't start to improve in a day, you need a vet visit, unless you're CERTAIN it's yeast. If you know it's yeast, you can get some lotrimin drops (for athlete's foot) at the drup store, or even a cream for vaginal yeast infections, put a little in the ear(s) twice a day and keep up the cleaning. If it doesn't start to improve in a day or so, time for the vet.
Yeast infections are really common, but important to treat... a series of yeast infections can lead to scar tissue which can cause hearing loss.