Seafolk（海人族, Kaijin-zoku / 深海族, Shinkai-zoku) were a race of beings that originated in the sea.
The Seafolk were a violent race of ocean-based creatures with a goal to take the surface of the Earth for their own and make humans their cattle. They were large humanoid creatures that possessed various aquatic animal traits such as gills, tentacles, and fins.
Even the messenger of the Seafolk had a threat level of "Wolf." Combat Seafolk have been shown to be powerful enough to lift a small truck with one arm. Several of them together were even more of a threat, with a threat level "Tiger". A group of Seafolk has been shown to be able to push a mid-ranking A-Class hero to his limits.
The first Seafolk rises from the sea and starts to head inland, but before he could even leave the water, he is destroyed by Saitama. Sometime later a large wave hits the shore of J-City carrying several Seafolk with it. The Seafolk then rise from the water and begin to wreak havoc in the city. The hero Stinger shows up and saves a citizen from the grip of one of the Seafolk.
Lord Great White, the last survivor of the Seafolk invasion eats the "King of the Lake". But afterward, Saitama quickly takes care of him.
During the Seafolk invasion, a shrimp-like member of the species is backed into a corner by the Blizzard Group, Lily tries to go in for the kill, only for the Seafolk to attack her with Acid Spit. Fubuki appears and disperses the Acid Spit before it can hit Lily and then proceeds to kill the Seafolk.
Webcomic and Manga Differences
In the manga and anime, the Seafolk attacked by the shore. In the webcomic, it was in the city. 
- (The Seafolk messenger to humans) "We are messagers from the deep sea! Humans! Surrender the Earth to the clan of the Seafolk!"
- Lord Great White appears in the opening sequence of Season 2 of the anime.
- According to Pig God's databook entry, there is an urban legend in which he fought 50 of the Seafolk by himself and wiped them out without leaving a trace.
- The Seafolk appeared to be a civilization developed enough to have its own concept of politics.
|Seafolk||Deep Sea King † • Lord Great White † • Messenger of the Seafolk † • Urn Eel †|
|Skyfolk||Sky King † • Eagle † • Hawk † • Falcon † • Kite †|
|Subterranean People||Subterranean King †|
|Terror Lizard Clan||Ancient King †|