My friends study the history of religions and say that, once Zeus was known for being the head of Olympus, while Poseidon was considered the chief deity. One piece of evidence they give for this is the trident.
Is this true?
Zeus, for the most part, seems to be represented as the head of the gods in most cases. For example:
Although Zeus is generally portrayed as king god, remember, there is no "set in stone" version of the myth. Since the Bronze Age, they were generally told orally from one person to another, each embellishing and carving it to their own liking. The version of the myths as most people today know them were described (and, importantly, written down) by authors such as Ovid, Homer,and Hesiod. While it is likely that their version of the myths was similar to everyone else's, there is no reason to believe that everything had to be exactly as we know it. A lot can change in a few thousand years! Sure, something like "who's chief of the gods" is a mighty big thing to change, but as you mentioned, it would actually make sense for Poseidon to be the chief deity, even if Zeus was technically king. Here's why:
Most of their world, if you notice, was ocean, or more specifically, Oceanus, the personification of the sea that supposedly surrounded the world.
From all this, it would make sense that the ancient people favored Poseidon over Zeus.
Note that I didn't mention the trident. That's because during a battle the Cyclopes gave each of the three elder gods a gift. Poseidon got his trident, Hades got the helmet of darkness which can make him disappear, and Zeus was given the lightning bolt. So they each got something to show their power and Poseidon's trident had nothing to do with it.
Poseidon may well have been a chief god in some particular places. This would have been in the Mycenaean period, when Poseidon had important cults at Pylos and Thebes. Also, on the Linear B tablets his name is more common than Zeus', along with a feminine variant that was probably his spouse. Other tablets record offerings to Poseidon and the Two Queens (assumed to be Demeter and Persephone).