I don’t think you’re going to find any earplugs that are going to be an adequate water barrier. My recommendation would be to use a high quality ear cleaner after each trip to the pool. Some cleaners are labeled as having a “drying action” (not sure what that would be), but any high quality cleaner should work. Just squirt a generous amount into ear ear, massage at the base of the ear for the nice squishy sound, and then let the dog shake his head (no need to use cotton or q-tips).