Last time I checked most motorcycles are intended to be ridden outdoors, and one of the nasty things about mother nature and the out-of-doors is that chit like rain, snow, sleet, wind, etc. tend to happen, often without warning. I realize that Harley has had a history of issues with their bikes and water (like electrical connections that weren't waterproof), but I've never seen them come right out officially and tell the customers to only ride in dry weather. So I'd say wash it the way you wash the rest of the bike and if the seat foam becomes soaked take it back to the dealer and insist they replace the defective seat. There is supposed to be a plastic barrier between the seat covering and the foam, and I have a sneaking suspicion that the report from another member about his foam being soaked had to do with that barrier being mispositioned or completely missing.
Jerry