My terminology:
- A sea plane is an airplane with an airframe (fuselage) that has been explicitly designed to float when it is in water (as well being designed to take off from and land on water).
- A float plane is typically a conventional airplane where the landing gear of the original design (e.g., wheels or skids) has been replaced with float gear for floating on water (as well as for taking off from and landing on water).
That is, they are similar, but not synonymous.
P.S. A couple of years ago, I got to sit right-seat on a flight of a Kenmore Air float plane from Victoria to Seattle. Way cool!!!