While it is true that the RN refer to submarines as boats it does not mean that all boats are submarines, viz ship's boat, e.g. pinnace, tender, cutter gig.
I wouldn't get too hung up on RN customs, they have some that are passing strange.
My understanding is:-
Seaplane - can land on water
Flying boat - derives the bulk of its flotation from the hull
Floatplane - derives its flotation from floats
Incidentally, anyone know the meaning of the expression "Getting out at Fratton"? A virtual pint for the first correct answer.