In order for Frontline flea medication to be most effective, you should not let your dog or cat swim, take a bath, or otherwise get wet until the medication has thoroughly dried on the skin which takes up to 24 hours.
Allow time for the medication to be wicked into the hair follicles of your dog. If you allow your pet to swim before this happens, the pesticide can be washed away and it will be less effective than it otherwise would have been.
After the application of Frontline has dried, however, you are free to let your dog or cat swim as much as they like because it will not make Frontline less effective at killing fleas, which is one of its great benefits.