In terms of water type, the standard ones are...
-Starmie
-Swampert
-Vaporeon
-Milotic
Starmie is fastest out of the 4 water types, and it can sweep or be used as staller, or even an utility Pokemon with thunder wave/confuse ray. You got psychic/thunderbolt/surf/ice beam at your disposal for sweeping purposes, and there's recover and cosmic power for defenses. You can also get a mix of them all. Starmie has a 360-370 speed which is very threatening.
Vaporeon can water absorb, and its best ability must be the high hp along with the good sp.def. It is the only one who can haze as well. Its special attack isn't low at all either. Its water absorb ability can crush many other water types (well except for Swampert and Starmie), especially Kingdra. The move wish also makes it a helpful teammate to have for healing the team.
Milotic has the highest special defense, and is very annoying. It has many stalling moves, such as confuse ray, hypnosis and most of them packs toxic as well. Mirror coat can put the end to Zapdos (it will survive that STAB thunderbolt easily), and also there's recover to make matters even worst for your enemy. Marvel Scale gives it a great advantage against any Pokemon who knows thunder wave. At the sacrifice of all of your speed (but then again, as if you have any) and perhaps a few skipped turns, you have both of your defenses at the 330s, with a 300+ max hp. Milotic's surf/ice beam isn't at all that weak either.
Swampert is the most offensive, and IMHO hardest to use because adjusting its EV seems to be very difficult (at least for me). A good swampert has decent defenses in both categories (unless you're against grass moves). It does a great job as a mixed sweeper, with a possibility of surf/ice beam/earthquake/brick break/rock slide all for you. It's hard to use, but once you got the hang of using Swampert, the results are fantastic.
As for Grumpig, I always think that it's a hard Pokemon to use due to low stats... I personally like Grumpig as an annoyer/utility Pokemon instead of sweeper (if you want to sweep, why not just take Alakazam? So might as well give it a use that no other Pokemon except Kecleon can do)
Grumpig- [sp.def + / atk -] @ choice band
EV: Speed, Sp.def, hp
-trick
-confuse ray
-light screen
-psywave
Trick the band asap, and unleash confuse ray. With their attack power to the max, confuse ray will hurt the opponent a lot. Set up light screen for the team, and it'll most likely force the banded special sweeper to switch out. Psywave is used as an offensive move that requires no EV investments in the attack category.
