The Seaking at the Cerulean City Gym has only had one appearance in the anime but nonetheless, made a difference in The Misty Mermaid, the episode it appeared in. It was in an underwater ballet written by Violet, one of Misty's three sisters.
At the beginning of the play it was seen among the Gym's and Misty's other Pokemon in the giant tank in which Gym battles and the ballet took place. It swam around Misty and danced until unfortunately, Team Rocket stole the show.
After Misty's Horsea used a Smokescreen attack against Jessie's Arbok in order to evade its Wrap attack, Seaking, without warning or command, came speeding through the thinning Smokescreen and used its Horn Attack on Jessie's Arbok. After a brief moment, Misty ordered Seaking to use Horn Drill on Arbok. Along with Misty's Starmie and Ash's Squirtle, Seaking triple teams Jessie's Arbok until Misty's sisters' Seel finally evolves into Dewgong and saves the day, freezing, and eventually defeating Team Rocket. Seaking never makes an appearance after this and most likely remains at the Cerulean City gym.