I've used Zymox myself with good results. Both of my dogs used to suffer terribly from recurring yeast infections in their ears.
One thing I did notice was that whatever I tried, despite nearly always clearing up the problem, it kept coming back. In the end I experimented with their diet and anti yeast foods. Incredibly since then both dogs have been yeast free.
I think the trouble is with yeast is that it's ever present and in the case of my dogs my vet said they were most likely allergic to the dog food I was feeding them, as soon as I changed this, well that's not strictly true, I tried a number of different foods, but in the end my vet explained about certain foods being better than others etc.
I must admit though in your case Back40, the fact you've had the dog a year and a half and it's the first time it's happened it could well have been debris in the ear or as you suggest the result of bathing. Dogs ears are notoriously bad for getting water in their ears, they're also designed perfectly for yeast to breed in.
I'm sure the Zymox will do the trick.