I honestly believe that Seaking is in a great spot right now, with of course its primary niche being as the single best counter to Rotom-F in the tier, with anything Rotom-F can do being offset by between Protect adding extra turns of Leftovers and the option of running Icium Z to block Trick completely and to grant recovery with Z-Haze. Its performance even outside of Frostom is stellar as its utility has a purpose in disrupting many defensive and offensive Pokemon, and having access to Knock Off in particular and being able to use it comfortably is such a valuable trait in this meta when you consider the prominence of threats like Mareanie, Bronzor, or Tangela. Seaking can also check other offensive Pokemon well like Zebstrika and Rapidash, two fast offensive threats that threaten a lot of slower bulky cores due to their speed and coverage, giving it plenty of options for a defensive backbone to bulky cores needing to fulfill these holes. While its defensive sets are rather passive, there exists a limit in which one can punish its passivity, especially when compared to Bronzor or even Mareanie due to Scald, Toxic, and Knock Off also giving it some mileage in any situation. The other important thing to consider is that when I look at other B- pokemon, it's debatable whether some of their niches are even comparable to consistently countering the best Pokemon in the tier that's also the most consistently offensive threatening to ZU teams, as well as it countering this Pokemon in a way that even Mareanie lacks against Monferno (blocking pivoting move). There are very few, if any Pokemon in the B- or even B rank, that has a niche that equates to an attribute of that level. For these reasons, I think Seaking has more than earned the opportunity to rise from its ranking.