Seaking

Swift Swim
Speed doubles in rain.
Water Veil
Prevents burn.
Type Tier
Water UU
Statistics
Min- Min Max Max+
HP
80
- 301 364 -
Atk
92
198 220 283 311
Def
65
149 166 229 251
SpA
65
149 166 229 251
SpD
80
176 196 259 284
Spe
68
154 172 235 258
Name Item Nature

Choice Band

Choice Band Adamant
Moveset EVs
~ Waterfall
~ Megahorn
~ Return / Double-Edge
~ Hydro Pump / Peck
4 HP / 252 Atk / 252 Spe

Choice Band takes advantage of Seaking’s (considerably) better attacking stat. Waterfall being physical is an improvement, however it came at the expense of losing physical Hidden Power. Megahorn is a nice, unique move on a Water-type that gives it at least a hint of worth. Normal moves complement Water very well for type coverage, and while using recoil moves on a Choice Bander is usually not a good idea, if you’re using Seaking you have already established your lack of rationality, so you might as well go for the extra oomph. Hydro Pump and Peck are both filler moves which you are unlikely to ever use. Hydro Pump can "hurt" Skarmory and the like, while Peck's only use is killing Shedinja.

Either ability is fine here. Swift Swim lets you take advantage of any lingering turns of Rain Dance, while Water Veil lets you switch into Will-O-Wisp and stops Seaking being crippled when switching into Flamethrower and such, if the secondary effect kicks in.

Name Item Nature

Flail

Liechi Berry Adamant
Moveset EVs
~ Substitute / Endure
~ Agility / Rain Dance
~ Flail
~ Waterfall
252 Atk / 4 SpD / 252 Spe

Boost your Speed, then use Substitute or Endure to get down to 1 HP. Liechi Berry kicks in, now you can attempt a sweep with a full powered Flail. Waterfall covers Rock-types and Ghosts that wall Flail.

Other Options

Other options include every single Water-type in the game not named Lumineon or Luvdisc. As far as moves go, Seaking learns the usual Water and Ice moves on the special side, along with the odd Psybeam, but it has a horrible Special Attack stat. Choice Scarf can replace Choice Band on that set, but that can be said for almost any Pokémon, and Seaking does not have the raw power to sweep without a boost. If OHKO attacks are allowed, then you can throw in Horn Drill. Aqua Tail provides some extra power over Waterfall, if you don't mind the less than perfect accuracy.

EVs

Max Attack and Speed. Seaking has an odd base Speed of 68, and defenses not worth writing home about, so trying to figure out EV spreads to outspeed this or that isn't worth it. After all, you’re using Seaking.

Opinion

Oddly enough, as absolutely abysmal as Seaking would be in OU, it could fare respectably in Ubers. On a rain team with Kyogre, it can abuse Swift Swim and throw around Choice Banded Megahorns in an environment that is largely Psychic. Unfortunately, it falters against the ubiquitous Giratina, Rayquaza and Dialga.

Seaking is fairly horrible though. Of course, in Pokémon, there is an inverse correlation between the quality of a Pokémon and the amount satisfaction you get from beating someone with it. Let that be your reason to use Seaking.

Counters

Surprisingly, finding safe switch-in is difficult, as the Choice Band set packs plenty of power in the lower tiers. Shedinja is your best bet, since he is immune to everything barring Peck. Pelipper works quite well as it resists both Waterfall and Megahorn, and has a good Defense stat and Roost to absorb Returns. Grass/Poison-types like Victreebel and Vileplume are also good options thanks to their neutrality to Megahorn.