If you want to run an offensive swampert i advice choice band (it two hits ferrothorn with stab earthquake after rocks dmg, also twohits jellicent)
movepool would look like this:
Move 1
Earthquake
Move 2
Waterfall
Move 3
Superpower
Move 4
Ice Punch
Stone Edge
ice punch obviously for drakes but since celebi isnt in the game it loses alot of priority while stone edge is mainly for hitting gyarados which is extremly popular.
EVs
172 HP
252 Atk
84 Spe
The ev's are made to let him hit the hardest possible (adamant nature) , while 84 speed are used to outspeed minimum speed scamory which allows you to 2ko it with waterfall in rain and the hp is given for more bulkiness.
tbh since i dont think drizzle (politoed) is in the game (again idk)? i would look for magneton with hp fire as teammate to trap ferrothorn/scamorys before they wall swampert
and at last: Swampert in ou loses alot of value since walls like gliscord,slowbro and hippodown are in the game which take away from his position (they are just simple better at it) but this set allows it to be pretty effective (with the right support) regardless. If you want to play defensive swampert then i advise to play the 3 pokemon i mentioned before.