Ships, buoys, Search-and-Rescue (SAR) aircraft, and shore-side base stations send out automated messages to let each other know where they are. Knowing where things are is useful when you want to avoid running into them :) They do this over VHF frequencies using a protocol called AIS. It was based on a similar protocol used by airplaines, called ADS-B.
For about 7.5 years as a government contractor, I worked on shore-side systems for streaming and hosting this data. This also involved web apps for querying and analyzing the data, monitoring the health of the system itself, and providing certain users the ability to broadcast text messages to vessels.
Ask me anything!
Cover image: Painting of the 5-passenger Ever Charming in the Elbe River near Hamburg, by Heinz G Klug. Source.