It doesn't need TSA, but if I'm correct it assumes you have a FAA license to base that off. I think some people have done seaplane ratings that they've later managed to endorse through CAA and to their JAA and then it's initial training and not an add on, so you would need TSA.
I could be wrong, but this is how I've come to understand it.
You are quite right. I did my FAA Commercial Seaplane at Jack Browns and then added it to my JAA CPL by doing initially the private seaplane exam at Ontrack and then the Commercial exams at Gatwick. All very painless and easy to do. Keeping it current is done by experience by doing the required 12 take off and landings on water. Easily done if you visit Jack Browns every couple of years.