No one's answered this yet? I'm surprised.
I think all types are covered by Water, Fighting, Ice, Ghost and Electric and Flying.
Assuming all of your Pokemon have a move the same type as themselves, one of the Pokemon on your team will be able to defeat the opponent in one way or another. :3
Anyhow, you've obviously got the Water-type covered, so I'll leave that.
For Fighting, you could try using a Lucario. It doesn't have a huge movepool, but it does have Fighting moves, if that's the most important thing to you. And the best part is that it can be found very early in-game.
There isn't a very big range of Ice-types available, so you'll just have to rely on getting another Pokemon that can learn Ice moves. Some Ice Pokemon are Vanillite (and evo line), Cubchoo (") and Cryogonal, but they are found rather late in the game, but not too late as to be past Opelucid City gym (I know Cubchoo isn't - it can be found just above Driftveil City in the Winter)
You can check here for Ice-type moves. (http://pokemondb.net/move/all)
Ghost is probably the most obvious choice - Litwick! I regret not getting Litwick in my game, because it's final evolution has a beastly Special Attack.
And Chandelure can then learn some pretty powerful Special attacks like Fire Blast or Overheat (I think it can learn Overheat if you go the Move Relearner, I'm not sure) Plus, a Fire-type is all too helpful.
As for an Electric type, there are plenty of them available such as Zebstrika, Ampharos. Ampharos is a Special Attacker, and Zebstrika is an Attacker. Those are my recommendations, so you can choose what you like better, I guess. xD
But take Eelektross is another decision to think. It's good in both the Attack and Special Attack stats, and it is not affected by anything. Since it's only weakness is Ground, and it has Levitate, it has no weaknesses. Seeing what type of team your wanting, I'd think you'd like this one. Tynamo can be pretty hard to find though, and in the beginning Tynamo are sorta powerless. Tynamo's highest base stat is 60 in Speed, the rest are rubbish. But if you train it with an Exp. Share or somethin', good things will happen. ;)
Finally, your Flying type is the Pokemon that will fly you around the Unova region.
There are so many good Flyers in this game, it's not funny. However my recommendation would be Flygon (no, not because I've used in the two times I've played the game, and because it's Hoenn, and because it's Dragon) because it's Ground/Dragon combo is great, compared to the normal Flying type. I'm not saying that Flygon is the only good flyer out there, there are plenty of other good ones, just search them up! :3
Okay, that's all. Have fun planning your team!
Hope I helped. :)
Source: Type Chart on the DB and Experience