As this for me is a recreational rating, can I assume that M1 Visas do not apply?
My PPL was a recreational rating and I still needed to get an M-1. So, no, you can't.
The criteria for needing an M-1 visa is whether your primary purpose of visiting the USA is training/study or not. Recreation or professional has nothing to do with it.
A floatplane rating doesn't need TSA clearance though, since it doesn't significantly enhance your ability to fly an airliner into a building.
Do you want to add the rating to your FAA PPL or to your JAA ATPL? I hear a lot of good stories from people who went to Como lake in Italy to get their floatplane rating added to their JAA license. And I don't think any of the JAA schools in the USA do floatplane ratings. OTOH, if you want to add it to your FAA PPL, plenty of choice.