The Deep Sea King (深海王, Shinkaiō) ruled over the Seafolk as their king, and claimed the deep sea as his own. He is the main antagonist of the Sea Monster Arc.
Abilities and Powers
The Deep Sea King is an immensely powerful being, able to fight on par and defeat several S-Class heroes and S-Class level individuals. He also appears to be able to sense people's presence in every direction, even if distant.
Water Empowerment: When his body is hydrated enough his form changes to that of his real form. In this form, he is much larger and more monstrous. However, sufficiently strong attacks or long periods out of water cause him to exit this form.
- Augmented Strength: In this form he boasts greater strength, being able to smash through buildings with ease.
- Augmented Speed: In this form his speed increases greatly, as he is able to keep up with Sonic, one of the fastest characters in the series, with ease for a considerable amount of time.
Immense Strength: The Deep Sea King boasts an extreme amount of strength seen as how he was able to dispatch two A-Class heroes with minimum effort. In his weakened state he's able to greatly damage and send Puri-Puri-Prisoner, an S-Class hero, flying through a building a great distance away alongside other heroes such as Lightning Max, an A-class hero. He was even able to tear apart Genos's cybernetic body with relative ease as well.
Immense Durability: The Deep Sea King has incredible physical might, being able to easily withstand the attacks of several heroes ranging from C-Class to S-Class with little damage despite them occurring one after another with little rest in between. In his weakened state, he is able to take the powerful strikes of S-Class hero, Puri-Puri-Prisoner, head on without taking much visible damage alongside the attacks of Sonic, an S-Class level non-hero and S-class hero, Genos.
Immense Speed: The Deep Sea King boasts an exceptional amount of raw speed, seen when he was able to appear behind Lightning Max instantaneously, catching the A-Class hero off guard. He even managed to slightly guard himself against Sonic, one of the fastest characters of the series, though couldn't land a proper hit on the ninja until he entered his true form.
Regeneration: While shown to be highly resistant to attacks, the Deep Sea King is capable of recovering from his injuries at an extreme rate, as seen when an injury he received from Genos, an S-Class hero, healed while they were still fighting. However, this ability does have limits. As a powerful blow can kill him before he has the chance to regenerate. 
Acid Spit: The Deep Sea King can shoot dangerous spit from his mouth that is extremely acidic, as it melted Genos' metallic body almost completely.
Master Hand-to-Hand Combatant: The Deep Sea King is an extremely skilled combatant, capable of defeating two S-Class heroes and seems to have lots of combat experience, as seen when he lectured Puri-Puri-Prisoner how to fight properly, by putting much more effort into attacking an opponent with more intent to kill. He also seems to have a preference for combo attacks, attacking his opponents mostly with a barrage of punches.
Body Moray (体内ウツボ, Tainai Utsubo): He can release a long eel-like creature from his mouth that he can use to bite and tear into opponents with. According to the Deep Sea King, the creature will not go away once it touched his enemies. Its full capabilities are unknown.