Honestly, Seaking is awful. While he has the fifth highest Attack of any Water Pokémon and decent HP, his other stats are mediocre. Also, the fact that there are so many Water Pokémon to choose from forces it to be cast aside for more useful Pokémon. His barren movepool makes him easy to counter as well. At least he can damage low Defense Pokémon decently!
This is a pretty simple set. Surf is a STAB move, Blizzard is useful against Grass- and Flying-types, and Double-Edge and Hyper Beam take advantage of Seaking's good Attack.
Substitute could help Seaking stay around longer to attack. Agility is another possibility, as it can overcome the Speed loss induced by paralysis. Mimic is useful in aiding his barren movepool. If OHKO moves are allowed, you can try Horn Drill. Seaking doesn't really have many extra options.
Checks and Counters
Gengar takes no damage from Seaking's Normal moves, and hardly any more from Surf or Blizzard. Starmie and Lapras switch in easily against Seaking and threaten it with Thunderbolt. Zapdos hits Seaking for super effective damage and is only 3HKOed by Blizzard in return; Jolteon fares even better against Seaking's Surf. Grass-types such as Venusaur are also good choices.
Seaking ends up fighting a losing battle against UU mainstays such as Tangela, who can recover off Seaking's damage with Mega Drain, and Golduck, who can Amnesia up while taking little damage. Electabuzz and Magneton both force Seaking to run in fear of a STAB Thunderbolt.