The Watermen were fishlike humanoids.
A group of Watermen from Q20 landed their ship after experiencing troubles with it. They came across the First Doctor, Ian Chesterton and Barbara Wright and introduced themselves to the group. The Doctor investigated their ship and determined that the fault of their ship was due to faulty wiring. After the Doctor made repairs, they embarked on a 5 million light-year journey back to Q20. (COMIC: Dr. Who Meets the Watermen)
Behind the scenes
The Watermen were one of the first examples of a humanoid alien species made explicitly for Doctor Who spin-off media.