The Deep Ones are a race of unknown origins, with two main theories, one stating that they evolved on Earth, and another stating they are alien in origin, much like their God, Cthulhu.
They are hybrids of fish, men, and frogs, with scales, bulging eyes, being bipedal in nature, with scales and frog-like coloration, their bellies beings off-white and their scales ranging from a light aqua to a dark green. Their mouths are home to sharp, needle like teeth similar to that of carnivorous fish. Their feet and hands both end in claws and possess webbing in-between the fingers and toes.
They have an alien culture to our own, possessing some governmental similarities of a dictatorship. They are ruled by Dagon, the great Father and Prince of the Seas, who is himself a massive Deep One. Due to evolving and living under the waves, without fire or metal, the Deep Ones' technology is truly eldritch in nature, but simple in mechanics, often using triggers of some form to activate a door, a weapon, or other things. A large portion of their technology is biological in nature, with their cities being grown, rather than built.
When the Deep Ones come from the sea to the surface, it is normally under the command of Dagon or Cthulhu, sometimes to fulfill the Oaths of Dagon, or to attack human colonies. It is unknown if Deep Ones reproduce through eggs or live birth, but due to the fact the "Innsmouth Look" is caused by the mating of a human and Deep One, there are strong arguments on both sides.
The Deep Ones, when they mate with a human female (which is often, if not always, forced) will father a child possessing the Innsmouth Look, which manifests as large arms, a slouch, large eyes, and quasi-gills. This child will slowly "evolve" into a Deep One, normally by the time they are 20-30, though it may happen much later in life.
Should someone with the Innsmouth look bear a child, that child is also doomed to become a Deep One. The Deep One "taint" seems to last forever within a bloodline, with people only very distantly related to a Deep One doomed to become one.