Carl Marin was the main antagonist of the Buffy episode "Go Fish" and he was originally the coach of Sunnydale High School's swim team but had sinister plans for his team to ensure they would become winners.
Carl Marin was employed as the coach of Sunnydale High School's swim team and was competitive man who wanted to win at any cost, sadly his obsession for winning was shown to go far beyond that of a normal sportsman as he was revealed to have been pumping the swim team with illegal steroid-like drugs via the steam room and as a result of extreme mutation caused by this drug many of the players became Gill Monsters (which were amphibious man-beasts similiar to the Creature From The Black Lagoon).
Rather than seek medical aid for his students and face responsibility for his actions, Carl opted for simply keeping his now monstrous players in an underground complex that he has access to via a trapdoor. When the school nurse discovers what is happening, she tries to stop Marin and save the students; unwilling to let her stop his plans, Carl tosses her into this trapdoor and the Gill Monsters devour her.
When Buffy arrives later to talk to Carl about the strange events that are going on, he pulls a gun on her, deciding she knew too much about his misdeeds (not realizing that Buffy was the Slayer), and forced her into the trapdoor, where he claimed the Gill Monsters would kill her. (He also implied that her death would not be a pleasant one with his comment that "boys have other needs".)
However Xander manages, by luck or fate, to arrive and disarm Carl before helping Buffy out of the underground complex - moments later Carl knocks Xander out with a pipe-wrech to the head and tries to do the same to Buffy but misjudges and falls into the trapdoor to meet a somewhat fitting demise at the teeth and claws of his own swim team (who were possibly angry with him for turning them into ugly sea monsters).
Following Carl's demise Xander (who had been a new recruit to the swim team) was given special treatment by Giles to ensure his exposure to Carl's drug would not have negative effects and the rest of the swim team (now Gill Monsters) start to swim out to sea..