FishMap.org has the ability to locate a watershed by entering your postal code or by clicking on a point on the map (which gets the postal code for that location). This would then query data owned by the EPA and return the watershed data by postal code.
Recently this information has been removed from the EPA web site and is not currently available. I have contacted them, but if this information is not returned and I do not find a substitute this feature may have to be removed from the site.
Any zip code that has been search before will still work because I was storing the data locally, but any new zip codes will bomb.
I will keep you updated.