When you are talking about the Hellenic Theoi or, in fact, deities from any pantheon, you will run into misconceptions about Them. Poseidon is not an exception to this rule.
The most prevalent misconception that people have about Poseidon is that he is “merely” the God of the Oceans. What this ignores is that he is one of the major deities that the Hellenic people relied on for much of their livelihood. Whether it was fishing or commerce, Hellenic society up until the modern era was heavily dependent on the oceans. He is a deity that the people of Hellas looked to for much of their safety, from both sea storms and from things like tsunami.
The reason for the Hellenes including the latter in their way of looking at Poseidon is that not only does he rule the oceans, but is also the deity who governs earthquakes, both in causing and in preventing them.
There is evidence as well that like his brothers Haides and Zeus, Poseidon had a role in the Eleusinian Mysteries. This is because of at least one altar found in his honor in the sacred precincts. This also ties him to the agricultural cycles and potentially to the life of the land as well as the sea.