From Star Trek Online GeekipediaPacifica, a world that is mostly water, with very few island masses. As a result the Selkie themselves have an unusual lifestyle, first being amphibious then as they grow older becoming fully aquatic only capable of breathing water for the rest of their lives.
As they are water breathing beings, special suits are created for the Selkie that keeps them hydrated at all times so their skin does not dry out as well as keeping them fully oxygenated. Most Selkie find this uncomfortable, however, and typically do not join Starfleet, but will use it when discussing with land breathers.
Pacificans themselves have been invaluable in oceanic research as being one of the few species that can dive deeply under water and explore the furthest reaches that modern conveniences have trouble with. Selkie often have a natural curiosity and are typically drawn to science, though because of their exhibited sense of freedom, they are also attracted to navigation, as with the USS Titan's navigational officer Aili Lavena.
In 2365 the Selkie were instrumental in bringing their fellow aquatic race the Antedeans into the Federation after two Antedean dignitaries were brought to Pacifica by the USS Enterprise-D.