There are two movesets what will be suit for Seaking, (in my opinion) enjoy!
Seaking @
Trait: Swift Swim
Adamant Nature
EVs: 252 Attack, 252 Defense, 4 HP
Moves:
- Rain Dance
- Waterfall
- Megahorn
- Poison Jab
(the EV spread is like this because, with this ability, with Rain Dance, it will double speed, and, also, it has bad defense so a little defense won't hurt)
Seaking @
Trait: Lightning Rod
Jolly Nature
EV: 252 Attack, 252 Speed, 4 HP
Moves:
- Waterfall
- Megahorn
- Poison Jab
- Smart Strike/Bounce/Throat Chop/Drill Run
(I recommending bounce because it will can defend from grass types but others is good too)