Explanation: The Roman Emperor Caligula, sometimes said to be mad, once ordered the Legions to 'make war' on Neptune, god of the sea, stabbing at the tides, and collecting seashells as war booty.

This may have been Caligula legitimately being unhinged. However, it may have been a punishment/humiliation for a failure to follow Caligula's orders appropriately, or simply the time-honored practice of autocrats making people do stupid shit not (only) for entertainment, but to showcase how absolute their power is. "Remember that I can make you do anything. And be afraid of what I can make others do to you if you forget that" sort of energy.

Of course, the TRUTH is that Caligula DROVE BACK that coward Neptune, and bought us almost 2000 years of uneasy peace with the ocean

The most likely explanation, IMO, is that it never happened. Historians made shit up all the time to tarnish the reputation of people they didn't like.

Caligula waging war on the sea is an almost 1:1 parallel to Herodotus's tale of Xerxes ordering the Hellespontus be struck with flails, which was a tale of hubris and foolishness. Herodotus's tale was much better though: Xerxes later went on to witness a horse giving birth to a hare. By contrast, Caligula immediately folded, declared it a win, and collected some seashells. As usual, Roman writers are bad imitators of the much more talented greek writers.

I wouldn't be surprised though since many if not most city Romans had serious lead poisoning from the water pipes.