Another interesting question is.. Can a CFI give instruction in a sea
plane even though he only holds a private sea rating (assuming the CFI
hold CFI-Airplane Single Engine) ? Back when my sea rating was only
private I asked our local FSDO office. They said they could not find
anything that said no but it didn't sound like a good idea (and I never
did it).
The FAR says you must have a commercial to APPLY for a CFI but not to
exercise it. This is a strange area because CFI is NOT specific to land
or sea, unlike a pilot certificate

-Robert, CFI (and commercial) land and sea