Swift Swim Sweeper (Seaking) @ Lum Berry  
Ability: Swift Swim  
EVs: 4 Atk / 252 SpA / 252 Spe  
Mild Nature  
- Rain Dance 
- Hydro Pump  
- Megahorn   
- Hidden Power [Grass] / Ice Beam

Bulky ToxTect (Seaking) @ Leftovers  
Ability: Swift Swim  
EVs: 252 HP / 64 Def / 192 Spe or 64 SpD
Timid Nature  
IVs: 2 Atk / 30 SpA  
- Toxic  
- Surf  
- Hidden Power [Grass] / Ice Beam 
- Protect
---------------------
Optional Move(s): 
- Surf
- Ice Beam
Optional Item(s): 
- Mystic Water
- Silver Powder
Optional Ability: 
- N/A
Optional EV/IV(s): 
- N/A
---------------------
Overview: Seaking stands out as the premier Swift Swim sweeper in the tier. While other options like Horsea or Luvdisc have higher base Special Attack and Speed stats respectively, Seaking's access to Megahorn and natural bulk as one of the few fully evolved Pokémon in the tier are what make it the most potent rain abuser. A more defensive set can also be ran with Toxic and Protect to make use of its high base Hitpoints, its respectable speed tier, and pure-Water typing. 
---------------------
Swift Swim Sweeper: Seaking is both the second fastest viable Swift Swim user after Luvdisc and has the second highest Special Attack stat after Horsea. It also possesses the highest base Attack of any Pokémon with the ability in the tier. As such, it is the prime candidate to take advantage of Rain Dance in the tier and wreak havoc. Rain Dance is mandatory on this set, even with additional dedicated setters such as Voltorb, as its natural bulk and typing allow the user to set up the rain both for Seaking and any teammates in the back. Its powerful Megahorn lets it severely weaken Grass-types hoping to tank a STAB Hydro Pump or Surf, and Hidden Power [Grass] lets you hit opposing Water-types super effectively.  
---------------------
Bulky ToxTect: This bulky alternative to its threatening sweeping set allows it to better handle the rain matchup, particularly versus opposing Seaking, as well as being a decent mid-ground switch-in to Pokémon like Growlithe and Rhyhorn. Its Speed with a Timid nature ensures that you are faster than max Speed Timid Growlithe. 
---------------------
Other Options: Players can choose Surf over Hydro Pump on the Swift Swim Sweeper set for more accuracy at the cost of less power. Ice Beam can be ran over Hidden Power [Grass] on both sets as a way to nail specially bulky Grass-types like Lileep, Bayleef, and Sunflora. However, on the Swift Swim Sweeper set specifically, Megahorn already accomplishes this and targets their typically weaker Defense stat. Ice Beam is a legitimate option for the Bulky ToxTect variant to target these same Pokémon, but then is walled by opposing Water-types, particularly the Seaking you are hoping to beat out with this very set. Mystic Water and Silver Powder are options over Lum Berry on the Swift Swim Sweeper set, but are generally outclassed by the utility provided by curing status that would otherwise hinder your sweep or clean.  
---------------------
Checks and Counters: Standard SpDef Grimer and Dustox can handle the Swift Swim set relatively well, although Dustox risks a potential 2hko from Hydro Pump in the rain, which also reduces the recovery from Moonlight, even if running Protect. However, both are great counters to the Bulky ToxTect set, as they take minimal damage from Surf, are immune to Toxic, and the former can even use the opportunity to set up with Curse. SpDef Grass-types like Bayleef and Sunflora can also check this variant, especially those opting for Hidden Power [Grass] over Ice Beam, but neither appreciates being hit with Toxic. Swift Swim Sweeper Seaking are checked defensively by Bulky ToxTect variants with Hidden Power [Grass].