What's it called when you break a champagne bottle on the side of a ship?

Christianing. It is a ceremony performed in reverence to god(s) so sailors and others on the ship travel safely across the sea. Historic records state that such rituals have been performed by humans as far back as 3000 BC.

The US Navy has an article about the history as well as the way other cultures around the world and across time have performed similar rituals. The article also states the different liquids also used.

American ceremonial practices for christening and launching quite naturally had their roots in Europe.

Champagne, perhaps because of its elegance as the aristocrat of wines, came into popular use as a "christening fluid" as the nineteenth century closed.

Whether for a massive nuclear aircraft carrier, destroyer, submarine, or amphibious type, the brief but impressive commissioning ceremony completes the cycle from christening and launching to full status as a ship of the United States Navy. Now, regardless of size and mission, the vessel and her crew stand ready to take their place in America's historic heritage of the sea.
https://www.history.navy.mil/research/histories/ship-histories/christening-launching-and-commissioning-of-u-s-navy-ships.html
 
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