Until a few years ago, 500 kHz was the one frequency marine radio
operators were REQUIRED to monitor continually, day and night, for
emergency traffic.

That is correct. I think that rule came in after the Titanic sank
(1912).

IIRC, the Titanic's sinking led to the creation of the International
Telecommunications Union, which was the first international attempt to
regulate and bring some sense of order to wireless transmissions. The
ITU was one of the most successful of the world's efforts at
international cooperation, which continued through war and peace.

vince norris