Arctozolt. A niche mon that has unexpectedly taken OU by storm, Zolt has brought itself and Hail to everyone's attention by the fact that almost nothing can safely switch into it. Ground types and Zeraora can't safely come in on its Blizzards, and Low Kick can destroy Ferrothorn and niche Mamoswine. Swampert and Gastro work fine- if Zolt isn't carrying Freeze-Dry (possibly good odds if one is not named okispokis). But, what if I were to tell you that a perfect counter (or at least a consistently good one) does exist, found in the obscure depths of ZU? I present to you:
Seaking @ Assault Vest
Ability: Lighting Rod
252 HP/140 Att/16 SpA/100 SpD; Sassy Nature
-Flip Turn
-Knock Off
-Drill Run
-Scald
You may have many questions, leading with what substances I am currently on. However, Seaking is in a rather unique position to counteract Zolt, shrugging off Bolt Beak and Blizzard being a mere 7HKO with Hail chip, with Low Kick only doing a max of ~25%; even surprise FD only does max 30%. While Seaking's Drill Run isn't quite strong enough to 2HKO reliably, its other options can make up the gap. Losing Boots makes Zolt sweeping a lot harder, Scald can burn and at neutral is just strong enough to break Sub sans Veil (while also preventing Freeze hacks), while Flip Turn allows you to pivot out into an offensive Zolt answer. Between these utility options and Drill Run, however, I've found that Seaking's role is not limited solely to checking Zolt.
While I specifically designed this set to check Zolt, Seaking has proven a surprising amount of worth as a special tank, while being very annoying to OU's scariest special attackers. For example:
0 SpA Heatran Earth Power vs. 252 HP / 100+ SpD Assault Vest Seaking: 53-63 (14.5 - 17.3%) -- possible 6HKO
140 Atk Seaking Drill Run vs. 252 HP / 0 Def Heatran: 240-284 (62.1 - 73.5%) -- guaranteed 2HKO after Leftovers recovery
176 SpA Slowking-Galar Sludge Bomb vs. 252 HP / 100+ SpD Assault Vest Seaking: 81-96 (22.2 - 26.3%) -- 10.6% chance to 4HKO
252+ SpA Choice Specs Volcanion Sludge Wave vs. 252 HP / 100+ SpD Assault Vest Seaking: 111-131 (30.4 - 35.9%) -- 44.8% chance to 3HKO
0- Atk Tapu Koko U-turn vs. 252 HP / 0 Def Seaking: 73-86 (20 - 23.6%) -- guaranteed 5HKO
252 SpA Tapu Fini Moonblast vs. 252 HP / 100+ SpD Assault Vest Seaking: 82-97 (22.5 - 26.6%) -- 23.8% chance to 4HKO
252 SpA Tornadus-Therian Hurricane vs. 252 HP / 100+ SpD Assault Vest Seaking: 103-123 (28.2 - 33.7%) -- 0.5% chance to 3HKO
252 Def Magnezone Body Press vs. 252 HP / 0 Def Seaking: 114-135 (31.3 - 37%) -- 80.4% chance to 3HKO
252 SpA Choice Specs Dragapult Draco Meteor over 2 turns vs. 252 HP / 100+ SpD Assault Vest Seaking: 259-307 (71.1 - 84.3%) -- not a KO
252 SpA Choice Specs Blacephalon Shadow Ball vs. 252 HP / 100+ SpD Assault Vest Seaking: 142-168 (39 - 46.1%) -- guaranteed 3HKO
140 Atk Seaking Knock Off (97.5 BP) vs. 0 HP / 0 Def Blacephalon: 250-296 (101.2 - 119.8%) -- guaranteed OHKO
As you can see, almost every relevant special attacker struggles to break through Seaking at neutral, allowing it to either take an item or gain momentum with Flip Turn. In terms of those attackers, Specs Lele is the only one that OHKOs, and even then only with Psyshock. Seaking doesn't necessarily break any of these besides Heatran and Blacephalon without chip, it's not supposed to; AV Seaking works as a tank that can generate momentum on special attackers while also forcing progress with Scald and Knock Off to chip away at key threats. As such, here's some things to note for teambuilding.
Corviknight is a great partner for AVking; Corv's physical bulk covers AVking's weakness to physical attackers, provides Defog support and blanks king's one weakness, while Seaking blanks both of Corv's weaknesses. Since it fits on most team styles, I'd say Corv is probably King's best partner.
Wish and Grassy Terrain can both help with Seaking's lack of recovery; WishPort Clefable has proved to be a solid partner, also potentially providing Rocks to add onto Seaking's brand of chip. Grassy Terrain can alternatively be used to negate small bits of chip (particularly Hail chip), with Rilla and Bulu both appreciating burns and item loss on opponents, not to mention slow pivoting support. If you can't fit both, Wish is better for defensive teams, Grassy is for more offensive teams.
Frail offensive mons always appreciate slow pivot support, and further appreciate King's blend of utility. This is aided by another benefit of Seaking (which I might be spoiling): the psychological factor. Nobody expects anything from a mon like Seaking, and I've found opponents tend to just give King turns to set hazards or attack it with their special attacker- assumptions that can allow in a terrifying teammate, and with mons like Weavile or Blacephalon that can potentially be game-changing. This makes Seaking a good gluemon on offense teams, especially since they don't care as much about King's lack of recovery.
In terms of other options, Ice Beam can be used if you really want to 2HKO Garchomp, Icy Wind can provide further support for slower breakers, Megahorn can OHKO Weavile (doesn't take +2 Knock though) and Whirlpool allows you to remove Glowking; regardless, you should at least keep Flip and Knock. Overall, Seaking's combo of utility, pivoting, Electric immunity and solid special bulk with AV allow it to serve as a gluemon for any team fearing the likes of Arctozolt, Heatran and Blacephalon. This is a VERY small niche, but one that does exist for the bravest of souls.
I had been meaning to find a proper use for Seaking for a long time now and I'm super stoked that you found one! Such an interesting Pokemon with a great movepool and ability. I also love the specialized EVs, some great stuff. It's also worth mentioning the Seaking set you listed works even better in Rain due to powered-up Flip Turns and Scalds, along with the utility of getting rid of Hail. It can also work in Sun to an extent under the same principle, as Seaking isn't focused on doing damage with its water moves as is, and sun can work to lure in Pokemon like Heatran on the opposing team, allowing mind games to get a potential Drill Run off, killing one of your opponent's most important team members.
I'd also like to add on here that Seaking gets additional great moves depending on specific needs for your team such as Megahorn, Haze, Scale Shot, Curse, Aqua Ring, Throat Chop, and Swords Dance.