It's not that Seaking is terrible. It's stats are rough, and mono-water isn't interesting, but Swift Swim turns Rain Dance into Dragon Dance, and it has pretty decent coverage in a lot of games.
Seaking's combination of mediocre stats, inconvenient availability, evolution level, and most of all atrocious level-up movepool are all factors towards its inevitable bottom placing for in-game usage lists.
The earliest available Goldeen is in the Hoenn games, where you can Old Rod for one at either Route 102 or ideally Petalburg City. However, RSE Goldeen has an absolutely godawful level-up movepool, learning not a single Water-type move until you get Surf, and unlike Gyarados, it doesn't have the raw stats to carry itself with unSTAB moves and doesn't even have the decency to evolve early; you're stuck with Goldeen's pitiful stats all the way until level 33, and the best thing you have to show for it all that time is "unSTAB Horn Attack" which won't even OHKO the likes of Zubats and Poochyenas.
While ORAS Goldeen's movepool improved slightly (getting STAB Water Pulse at level 16), so does literally every other Water-type in the game, with all the pre-Surf Water options like Lotad, Wingull, Magikarp, Tentacool, and especially
Marill all being vastly more appealing candidates. As the final nail in the coffin, Old Rod Goldeen also comes extremely underleveled at level 5; Old Rod Tentacool also is saddled with this issue but at least when it reaches its level 16 Water Pulse, it's not coming off a garbage 35 SpAtk stat like Goldeen's, on top of evolving earlier at level 30.