I'll rate Swampert.
Nature: Lonely (+ATK, -DEF) or Mild (+S.ATK, -DEF) or Gentle (+S.DEF, -DEF)
EVs: 252 HP, 132 Attack, 126 Special Defense
One of the best bulky water ever created was Swampert. If you can't/don't have one, you could catch a Wooper or Barboach and evolve them into Quagsire and Whiscash, respectivelly, but it's not the same of having a Swampert in team. Swampert is water and ground type, the only move type that damages it effectivelly is grass and it's four times the normal damage, so you should think twice before putting Swampert out to find against a grass type Pokémon. Let's take a look at his base stats:
Special Attack: 85
Special Defense: 90
It seems our Swampert isn't fast on attacking, so most of the times he'll attacks after foe. Generally, people prefer natures that lower the Speed, but I personelly think that lowering Defense is better once that Swampert has no physical weakness. His only problem are the grass type Pokemons, and being 4 times weak to grass seems enough for a weak point. The natures I suggest are Lonely, Mild or Gentle. Swampert is great on HP and Attack, good on Special Defense, Special Attack and Defense, and cheap on Speed. In my moveset HP, Attack and Special Defense were prioritized, as you can notice in the EV points I suggest you to collect, as well as in the moveset I built for it, which shall to be used along.
Building this set of moves aren't that hard.
Surf is HM03
Muddy Watter is learned at Lv39.
Ice Beam is the TM13.
Blizzard is the TM14.
Earthquake is learned at Lv46, but you could waste the TM26 on it.
Protect is learned very early, at Lv17.
Attract is the TM45.
This is good, we don't need to run around very much looking for any move-tutors once Swampert can easily learn of my suggested moves by TMs or Level Moves. No Egg Moves. Some people say that Hydro Pump should be included, but that's only for Mudkip and it's up to you decide to level it up to Level 42, consequently you get Muddy Water afterwards only with the help of Move Maniac in Fallabor Town, which costs 1 Heart Scale. You don't need both Muddy Water and Hydro Pump, but in case you want to teach it over Hydro Pump, that will be the only available way.
Now, how to use Swampert?
As a bulky water, his moveset was built to make him a special sweeper although 25% of the EVs were directed to Attack and not to both Attack and Special Attack. Swampert got two STAB moves, Surf and Earthquake, which have to be used all the times, except when Ice Beam causes super effective damage or if foe is grass type. The fourth move of the moveset is a defensive move, I think it's good to have one, you can teach another move if you prefer by your own way.
*Surf of Muddy Water? Surf fail less times than Muddy Water, but it doesn't have any effect like Muddy Water, which is 30% chances of lowering foe's Accuracy in one level;
*Ice Beam is a lot more accurate than Blizzard, but doesn't cause all that freezing;
*Protect or Attract? If your Swampert is female, teach Attract. If not, teach Protect. The good of Protect is that it will give a free turn for Leftovers to recover your HP. there are many moves you can teach instead Protect or Attract: Brick Break, Swords Dance, Endeavor, etc.
*Fighting against grass type Pokémons is about to you decide. A simple Giga Drain can take it down.