Sea Devils looked somewhat like humanoid turtles or plesiosaurus. They had beak-like mouths, large eyes, two fins on their head and a long neck. They had three webbed digits on their hands and feet. Their skin-tone came in a variety of shade, including green, blue and orange. They were much tougher then humans, since they need to survive the crushing depths of the ocean. Sea Devils communicated in a language like sonar, but they could speak human languages in a whispering, rasping voice.
The Silurians were related to the Sea Devils, though they were genetically divergent enough to create problems for the offspring of such matings.
The Sea Devils were more advanced than the Humans of the 20th century when they were forced to go into hibernation.
The Sea Devils had millions of underwater colonies, filled with thousands of Sea Devil. These bases appeared to be carved out of rocks and a generally covered in algae and seaweed. These colonies could be reactivated en masse, usually with a sonic signal. They use a less effective method hibernation technology in these bases, leading to some muscular degradation when they were brought back. They also used special orange capsules to transport land-dwelling creatures.
The Sea Devils had large supplies of weaponry in these bases. They typically used a small, round, side-arm. During their invasion of Sea Base 4, they made use of a larger version of the device,to cut through the doors.
Common Sea Devils wore a type of blue, gauze-like dress. The soldiers in the 2084 invasion, being elite soldiers, wore thick black armour somewhat reminiscent of samurai armour.
While the Silurians ruled the Earth, the Sea Devils were used as foot soldiers. When the Moon was coming dangerously close to Earth, they also went into their colonies to hibernate. After millions of years, some of the colonies were awoken.
They were first awoken in the mid 20th century by the Time Lord known as the Master. He attempted to persuade them to reclaim the planet from the human race. Despite the Doctor's efforts to convince them otherwise, the Sea Devils eventually decided to go to war, and the Doctor was forced to destroy their base.
A different colony of Sea Devils were awoken in 2084 by the Silurians. They were sent as shock troops to invade Sea Base 4. Though they were better prepared, they were all killed by the Hexachromite gas used by the Doctor.
At some point in the distant future, the Silurians and Sea Devils formed a lasting peace with Humanity. In this era, a number of Sea Devils as well as Humans colonized the planet Chosan which was being attacked by Calabraxian terrorists. This prompted the Earth Reptile Council to ask the Braxiatel Collection for aid which they sent in the form of Bernice Summerfield. Her time there revealed that the ocean planet possessed a sentient protein in the waters which had began to infect the water dwelling Sea Devils. The entities attempted to possess the Sea Devils and use them as an army against the Humans but faced difficulty in integrating themselves into the host biology which led to a number of mutations. They were ultimately successful and had managed to possess the Sea Devil Lurnix as a host who they attempted to use to lower the security grid thus infecting all the Sea Devils to use as an army. However, Bernice Summerfield was successful in evacuvating the colony and detonating a bomb at Chosan thus ending that threat. As a result, the Sea Devils were forced to abandon the Chosan colony due to the threat posed by the alien protein.