Poseidon's domain is the OCEAN, SEA, and thesea. Poseidon was the Greek god of the SEA, earthquakes, and horses. He was also known as the 'Earth-Shaker' due to his power over the seas. His domain included the entire OCEAN, as well as the smaller seas and theseas. The OCEAN is the largest body of water on Earth, covering nearly 70% of the planet's surface. The SEA is a smaller body of saltwater, usually located between landmasses. The thesea is an even smaller body of water, often found in coastal areas.