The full body of the Sea God is never shown, but it is massive enough that it destroyed the island housing it in the process of escaping in order to pursue prey. If a rough estimate is to be made, its total mass may equal or rival that of Shiva. The top half of it that sits above the water consists of a massive dorsal dome, the base of which possesses a massive maw lined with many rows of jagged teeth, some large and some small. Against its palate and the base of its mouth, along with its chin area are hundreds of tentacles, each capable of attacking individually. If a comparison were to be made, the general mass of the Sea God could be likened to a jellyfish.
Its insides were massive enough that they could be navigated like tunnels, and so much blood filled its body that slashing any one wall open presented a serious threat of drowning, even when the god was dead and deflated. Within its digestive tracts were many capsized ships, swallowed whole throughout the ages of the Sea God's existence.
Its heart was roughly twelve feet in height and possessed dozens of eyes along its surface, each of which would open if anything came close enough to attack.
The Sea God has innumerable tentacles that extend from every side of its body, each tentacle branching into several more. The Sea God can transform those it devours into tentacle-based extensions of itself, allowing them to maintain their sense of individuality as well as their natural form (take for example, Bonebeard). These tentacles could be separated from the main being while still under its control, allowing it to send drones out to survey and attack.
Though not an ability in its own right, the Sea God's innards housed thousands of creatures both symbiotic and parasitic, which tended to attack foreign objects like Guts and Schierke, acting as a line of defense. These consisted of fairly large spider-like creatures roughly two feet tall.
Its final line of defense involved its massive size and power. Its heart and cardiovascular system is essentially a sonic weapon, launching the waves of kinetic energy along with its inner muscle necessary to keep it alive, but also consciously weaponizing this fact. These sound waves were such that a well-built man like Guts could barely endure them and could not approach close enough to attack, quickly becoming dizzied and losing sense of direction and eventually consciousness, possibly cracking his skull and causing several other definite internal injuries.
Long ago, the Sea God consumed all matter within its reach before being sealed within an island by the Merrow. Since then, the Sea God was kept fed by its worshipers, the island's fearful inhabitants. After the World Transformation, the Sea God gained a firm grasp within the physical world, reclaiming its freedom and, save for Isma, consuming all the island's inhabitants as well as Bonebeard and his crew.
- The Sea God has many similarities with Lovecraftian entities. The creature's use of villagers who worshiped it before becoming inhuman is similar to the story of The Shadow over Innsmouth.
- The Sea God is also very similar to the Kraken, a mythical Nordic creature which appears to be an island to unknowing sailors, yet is an octopus of immense size.
- The Sea God's heart has a similar appearance to the Idea of Evil. Both have a heart with eyes.