So what does it take for a Flight Instructor (CFI) to teach in a sea plane or amphibious airplane? Is there any special or additional training requirements? Any CFI certificate endorsement required?

The answer is simple; you are allowed to teach in any single engine airplane for which you hold pilot privileges. Your CFI certificate says – “Airplane Single Engine”. Your pilot certificate (commercial pilot) reads – “Airplane single engine land, Airplane single engine sea”. In this case you can teach in both the land and sea single engine airplanes.

Piper Cub on Floats

In other words, if you already hold a commercial pilot and a flight instructor (CFI) certificate, and you go get a single engine sea add-on rating on your commercial pilot certificate, you are authorized to teach in a sea plane or an amphibious airplane.