I actually somewhat care about defending Seaking. It does some things right.
1) Recognizable design. That black and bright orange sets it apart, even today.
2) Color balance. The mottled intense colors at the center, with much softer white and specks of blue at the periphery, make it genuinely very pleasant to look at, dynamic and striking without being too busy and cluttered.
3) Shape. With its rounded, stouter, compact figure instead of a koi fish, it both looks more Pokemon-like and more active, more full of agency, more like it would strike or hit at something.
4) Transition. All these three advantages are more pronounced because Goldeen doesn't have them. Goldeen is paler, balances color differently, and, thanks to its tail, longer. A more basic fish design leading into Seaking make its good traits stand out and feel like an improvement.
5) The horn gives it something to do that isn't just swimming. I know a lot of Gen1 Pokemon have this horn somewhat arbitrarily, but it's especially great for Seaking (who has less obvious ways to interact and engage otherwise), and they've used the horn to give Seaking cool goodies like Megahorn, Drill Run, and Lightningrod.
6) After enough generations, it's in a pretty neat spot in battle, with Swords Dance, Swift Swim, Lightningrod, and a colorful coverage set letting it do a lot of cool things. It is far from good, but it is unique and cool, and that's what I care about in battle.
Seaking's no Gengar or Charizard, but I'm legitimately glad it's here. Personally, it's my 5th favorite Water-type and in my top 100 overall.