It's extremely doubtful that the infections in the ears are food related. As you note the frequency is only a couple times of year. If it were food the logic would imply the particular food causing the problem is being administered only a couple of times a year.

There is potential the other dog licking the Lab's ears might setup a moist environment suitable for bacteria or yeast, but again the bets are the other dog licks the ears of its companion more than just a couple times a year.

You need to consider seasonal allergies combined with humid conditions, and scratching. If your Lab swims at the times of year you are seeing the infection then you have the base cause.

You have 2 basic choices for treatment:

1. In the absence of infection swab the ears every other day or immediately after swimming with 1 part white vinegar + 1 parts deionized water. The vinegar drops the pH such that yeast can not grow.

2. If infected and prior to applying antibiotic salve then use a foaming cleaning and drying agent - precription from the Vet. This is followed by application of an appropriate precription ointment such as Ottomax.

If you notice the dog scratching its ears, try giving your Lab 50 mg of Benedryl at night, and another 25 mg in the morning until the allergy season passes.