We use Advocate but, given both Milly's and Chaz's love for anything even remotely moist from a bit of mud to the north sea, I checked up on Advocate's resistance to water.
According to our vet tests have been done by the manufacturers who say that, four hours after application, the treatment has been absorbed into the Lipid layer of the dog's skin and is not affected by shampooing, bathing, heavy rainfall or swimming.
Don't know if there are any limitations as to the amount or frequency of bathing but it seems to keep working for us, no fleas so far, right through the rainy season which is most of the year up here, when mud has to be hosed off almost daily
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Bob.