Sven wrote:

I think the international rules say that you fly the flag of the country
of registry (flag of convenience ?).

A flag of convenience is a political and economic construct. It has
absolutely nothing to do with the nationality of the owner and by
definition does not reflect the nationality or residence of the owner or
operator.

Is it improper (illegal) to instead fly the flag of the owner's
nationality as long as a courtesy flag is flown under the spreader ?

Yes, it is illegal. The ship must fly the flag of its nation of
registry. This is called the "flag state" for a reason.

A courtesy flag is the flag of the "flag state" within whose waters the
ship is sailing. The flag of the ships registry is never a courtesy flag
for that ship.

Related question; are there any maritime law web sites for future
reference ?

Yes, lots of them. Try Google for flag state, flag of convenience, IMO,
FOC. Go from there.

Rick