Yep you will need VISA, TSA and SEVIS authorisation to do the seaplane rating. It will be far easier to go to Scotland to do it. After all it is only 5 hours training and one ground exam. Plus all the UK seaplane instructors live there and the only two UK examiners run the only two UK schools there.
I am heading up there sometime between March and May to get th rating. Also in the UK if you do the Commercial sea plane rating you can pin that to your JAA PPL (even if you dont have a CPL yet). tha tis how I am doing it, that way when i have completed my CPL (later this year) I can automaticaly add my Sea plane rating to it.