My understanding of the nomenclature is that an aircraft that lands on floats, is a seaplane, whereas one that lands on its hull is a flying boat.
Ian you are absolutely correct regarding British nomenclature. However , on the other side of the Pond, "seaplane" loosely refers to any fixed wing type that lands on water.
Americans now also often use the term "aerostat", when actually referring specifically to what is a tethered aerostat. The "stat" part of the definition being misunderstood ( a dirigible airship also being an aerostat).
Aerostats as distinct from aerodynes seemingly now being little understood concepts in terms of their original etymology, judging by the long established adoption of the term "airplane".