Carl Marin is the main antagonist of the episode ''Go Fish'' and also the coach of Sunnydale High School's swin team, he also had many sinister plans to turn his students into humanoid marine monsters.
Do not be too much about Carl Marin's past life, as it is known that he was a swin coach who worked in Sunnydale. He somehow figured out how to mutate humans, and gave illegal steroid-like drug, which actually turned them into Gill Monsters, similar to Gill-man. However after the coach causes many murders indirectly and makes them attack people. Buffy then realizes and attacks the monsters, Marin then quickly earns the trust of Buffy's friends. However, after he gets what he wants, Marin betrays them.
However, both Xander and Giles get special treatment from Marin. And in the end, the trainer is killed foir his own creation.