It's different across the pond!
Now my understanding, a bit rusty, is unless the lake is specifically prohibited, you can land on it in Florida. ( And probably in Alaska, and Minnesota, the land of 1,000 lakes)
The only thing is, everything on or in the water has right of way over your seaplane, including alligators, swimmers, waterskiers, whatever, as they do not understand your needs.
In general, watery surfaces are a lot more available in the US and Canada than they are in the UK, here it is ancient landlord and fishing rights that get in the way, and of course in the Lake District, the understandable desire for peace and quiet.
Problem with landing on the Thames might be that the authorities tend to frown on flying under bridges.....
Ireland, being less crowded, and having removed the English Landlords, might be more welcoming to a seaplane operation?
But in the UK, alas, even the classic old seaplanes that used to grace our shores do come to grief, there was one in the Solent not that long ago, anyone remember the details?