Originally Posted by johnnysnow

This is correct...flight instruction towards an instrument rating, currency, or an IPC requires only the CFI-Airplane certificate, which is valid in any airplane for which you hold appropriate commercial privileges.

Flight instruction towards other certs/ratings and proficiency such as landings, BFR, etc requires a CFI Airplane with the appropriate numner of engines. But even the CFI-airplane is only partially class-specific...it specifies single or multi-engine but not land or sea. There is no seaplane instructor rating, you only need seaplane on your commercial and a CFI-airplane (SE or ME as appropriate).