During the early 1930's, there were several gambling ships anchored in
international waters off the coast of California.

At least two of these ships carried radio transmitters that were on the air
with broadcast programming.

In radio terminology, we would today describe these shipboard radio stations
as "pirates".

The "City of Panama" was registered in Panama, and its radio transmitter
emitted 500 watts on 815 kHz. The sal****er pathway assured a very strong
signal in the Santa Monica areas of Los Angeles thus causing severe
interference to legally licensed land-based stations. The impact of this
station, which was on the air under the unusual callsign RXKR, also caused a
flurry of diplomatic activity.

Another radio station during the same era and in the same area off the coast
of Los Angeles was on board the gambling ship, "Rex".

(Source : http://radiodx.com/spdxr/Ship_BCasters_Americas.htm )