Frontline plus is waterproof after the initial application has dried which takes about 24 hours. After that time, you can have your pet swim or given a bath and it will not lessen the effectiveness of the flea protection.
But you should not let your pet get wet in the first 24 hours after you give it Frontline or else the product will be washed away before it has time to be wicked into the hair follicles of your dog or cat and spread across the body by process called translocation.
In addition, you never want to apply Frontline to an animal that is not completely dry because it will also prevent the pesticide from being spread across the body and will severely reduce the effectiveness of it.