To get a seaplane rating in the US you must first get a validation of your foreign licence. It is free and can be done at any FAA FSDO. You must have a valid medical from the country for which you hold the pilot licence.
The ONLY certificate validation that the FAA will give you is a Private regardless of what other foriegn licences you may hold. i.e. An ATP from elsewhere will only get you a Private.
You can then go and get any FAA rating like a Seaplane or Multi-eng etc. Would SA validate a US rating, that you must ask them?