Seeming as how this is in-game..
For Water I would suggest Tentacruel, it's a bulky water/poison that you could give good moves such as -
Waterfall, Surf, Poison Jab/Sludge Bomb, Ice Beam.
For Fire try out the Pokemon Rapidash, it's fast and if you can give it Megahorn, Flare Blitz, Bounce and...