Twice-Told-Tales: Warring Water-Breathing Warriors

The dominant intelligent undersea races were the Atlanteans, the most prominent of these residing in the twin cities of Poseidonis and Tritonis on Earth-One as well as the those of Aurania and Venturia on Earth-Two. These were originally humans who adapted to their new environments... Poseidonians became water-breathers with dense physiology, Tritonians became mer-people, those of Aurania and Venturia became gigantic females and diminutive males. There were likewise mer-people on Earth-Two and altered-sized inhabitants on Earth-One in various smaller Atlantean colonies scattered throughout the seven seas of their worlds.

Two divergent species*, less populace than the Atlanteans, encountered humans above the surface on occasion... and posed significant threats to humanity in general. These were the Saremites, a pale-skinned humanoid civilization originally peaceful until threatened by outside entities.  And then, there were the Kogats, a furry beastial society that only knew of warfare when faced with similar outside entities

When first discovered, the Saremites were being invased by the Maugites. After rescuing an astronaut named Fred Jensen in his downed vessel which crashed into the ocean, they sent him back to the surface to find help for them. The Flash journeyed to Sareme, and repulsed the Maugites' attacking forces. 

King Jupo of the Kogats sent an invading force to the surface, kidnapping a friend of Carter Hall's named George who possessed a special weapon. Seeking to conquer the surface world, they found a worthy opponent in Hall's alter ego, the Hawkman, who was searching for his missing friend. Aided by an underwater being who granted him the ability to breathe underwater, Hawkman used a special device to destroy the Needle of Rock that supported the Kogats' city. The Kogats were apparently all destroyed.

A few years after their meeting with the Flash, the Saremites become more aggressive after the United States of America dumped toxic waste into the waters above their city's cavern, damaging their Proof-Rock which had guided them. King Nebeur Odagled sent his Saremites to attack Atlantis, whom they associated with surface humans. This led to Hawkman and his Justice League allies battling the Saremites. At one point, Hawkman nearly drowned until Superman revived him. Soon after, the League triumphed and restored their Proof-Rock. During the course of this adventure, the League attempted to locate Flash who previously met them, but he was left comatose on another case and only revived after the Saremites were defeated.

* Besides having the ability to live indefinitely underwater, the Saremites and Kogats have a form of telepathy allowing them to communicate with humans.