he drew lots with Zeus and hades to where they would rule
Poseidon did not "become" God of the sea. The Greeks essentially 'created' the Gods to understand their world. ex. Poseidon created the sea storms, Helios carried the sun up into the sky every...
after him, Hades, and Zeus threw their father (Kronos; lord of the titans) in tarterus, the 3 brothers agreed into it.
- anger - protector of all waters - moody
In Greek mythology, Poseidon was the god of the sea. If he was real or not is up to your belief, however, it is generally accepted as pure myth.