Bonjour, here is the general idea: An aditional rating given by the administration of country X can only be written on a licence issued by contry X.
i.e the FAA will add a rating on an FAA licence only so if you chose to do seaplane training in the USA you must get an FAA pilot certificate. The FAA canot give a "single engine sea" rating on a non FAA certificate.This means that you must get the certificate, not a validation of your SA license. The validation will allow you to exercise in the USA the privileges of your SA license.