All right. I won't include items in this, so you can pick your own.
SEIESMATOAD IS THE BEST OPTION! Even though you have a bunch of ground moves, it just wouldn't do to switch in Stache for EVERY SINGLE ELECTRIC POKEMON. Now, Seismetiod is the best water/ground you will even find tons of Tymepole. Also, for a plus, you can run rock slide or some good fighting move to counter freeze dry, which isn't very common, but it's nice to be able to cover ANYTHING that your opponent throws at you. For a plus, Seismatoad can be boosted by swift swim, so grass trainers(Milo) won't be a problem, when you throw sludge bomb or something into their faces.
Seismitoad
Ability: Swift Swim
- Earthquake(electric counter; stab)
- Liquidation(good water stab)
- Poison Jab(a bit better then sludge bomb cause Seismetoad has better attack then special attack)
- Drain Punch(freeze fry coverage)
You can use Seaking as a replacement, if you can't snag Seismatoad or if you just simply don't want it. I know thunder rod is a hidden ability, but you should be able to do it(eventually). The reasons this guy is a replacement is: thunder Rod burns out the ability slot, and Seaking can't really counter grass types. Still, I have no idea why they forgot Poor Seaking, although I wouldn't recommend it.
Seaking
Ability: Lightning Rod(without thunder rod, Seaking wouldn't be on this list!)
- Drill Run(electric counter)
- Megahorn(good staple bug move)
- Poison Jab(grass counter)
- Surf(100% accuracy water move(that's actually good))
"What about Gastrodon?" you say. Well, Gastrodon can counter Grass, which Seaking can't do, but it's stats are TRASH, which is a bummer. Seismetoad can at least dish out lost of damage when it's on the field. Gastrodon has good poison type moves, but poison type moves are generally special and with a good special attack, Gastrodon I'd good and all, but here's the thing: Gastrodon is so slow it'll get 0-hit-koed before it can do any damage. Seismetoad at least has a chance. Trust me, even SEAKING has better overall stats.
Gastrodon
Ability: Storm Drain(extra immunities are nice)
- Ice Beam(grass counter)
- Earthquake(electric counter; stab)
- Brine(to finish off weakened opponents; stab)
- Scald(stab)
Lanturn is (kind of) good. It can counter grass types, all right, but it's so slow it gets koed before it can do lots of damage, plus a base special attack stat of, um... 76? Come on! At least Lapras gets stab! Lanturn is good and all, but it won't do for a water monotype team that well. There are more and better viable options. But if you must use it, go for a special attacker or a rain & thunder combo.
Lanturn
Ability: Volt Absorb(without volt absorb, Lanturn wouldn't be on this list!)
- Ice Beam(grass counter)
- Scald(stab; good chance to burn)
- Brine(finish off weakened opponents; stab)
- Discharge(Thunder replacement with a good paralysis chance)
Lanturn
Ability: Volt Absorb(without volt absorb, Lanturn wouldn't be on this list!)
- Ice Beam(grass counter)
- Thunder(combo with rain dance)
- Brine(finish off weakened opponents; stab)
- Rain Dance(combo with thunder)
Quagsire. Sighs. This guy is absolute TRASH, but technically, it's viable. Barely. You should go for a more even set, with a mix of special attacks and physicals attacks.
Quagsire
Ability: Water Absorb(an extra immunity is nice)
- Liquidation(strong stab physical water move)
- Earthquake(electric counter; stab)
- Ancient Power(freeze dry counter)
- Sludge Bomb(another grass counter)
Whishcash is um... fairy decent, I guess. Exactly the same as most of the others, this guys pretty bad, but it's a solid choice because it has a fairly decent movepool and it's just pretty good overall. Unfortunatly, it's stats are pretty bad, but it still scraped into a good spot in the list.
Whiscash
Ability: Anticipation(the best ability for this guy, in my opinion.)
- Ice Beam(good grass counter)
- Earthquake(electric counter; stab)
- Spark(no one will see this one coming!)
- Any good move(Whishcash doen't really need anything else, but it's good to have a backup move)
list:
Best: Seismetoad
2nd best: Seaking
3rd best: Lanturn
3rd worst: Whishcash
2nd worst: Gastrodon
Worst: Quagsire
Note: If you can't get the exact movesets, then use a move of the same type that's at least decent.
Note: Whenever I say 'grass counter', I also mean coverage.
Hope this helps!