The Stars are Fire
Genetically engineered to live in the water oceans discovered beneath the ice crusts of various solar moons, natathim (Homo aquus) have human ancestors, but barely look it. Survival in the frigid, lightless depths of extraterrestrial oceans required extreme adaptation. Predominantly dark blue, their undersides countershade to pure white. Though humanoid, their physiology is streamlined, giving their heads a somewhat fish-like shape, complete with gills and large eyes to collect light in the depths. Their bodies are adorned with fins and frills, including a long shark-like tail, and they have webbed extremities with retractable claws.
Depending on the setting, natathim are either human allies with the same (or even more advanced) tech, enemies with the same or more advanced tech, or genetic anomalies treated like laboratory rats burning with genocidal fury at what's been done to them. Alternatively, natathim could be discovered in Earth's deepest oceans, their origin mysterious, but able to interbreed with humans as a method for maintaining their line.
Motive: Just as with humans, natathim have many and varied motivations and drives.
Environment: Anywhere in or near water, or in suits/craft with marine environments, in schools of three to twelve. natathim can act normally in air for up to twenty-four hours before they must return to water. (Ocean)
Health: 9
Damage: 4 points
Armor: 2
Movement: Short on land; long in the water
Modifications: Swims as level 6
Combat: natathim attack with their retractable claws or, if available, technological weapons. Some have a magnetoreception ability that allows them to see into frequencies other creatures can't, or even stranger abilities to interact magnetically with their surroundings, though this is little understood.
Interactions: natathim can be sympathetic to humans, partners in space exploration, or consider humans to be bitter foes for having created their species in the first place, depending on the setting.
Uses: The PCs find evidence of an illegal gene tailoring experiment, with evidence pointing to research being done somewhere in the Opulence of Outer Planets.
Loot: Some natathim carry valuable items and equipment.
GM Intrusion: The natathim spontaneously magnetizes the character's possessions, which hold them helpless against the nearest wall or floor (if also metallic). The PC can take no actions other than attempt to escape.