Yes regular tap water can be used in your Keurig.
I use nothing but regular tap water right out of my faucet to make coffee using my Keurig and it makes the coffee just as good as bottled water.
Bottled water can get expensive and the tap water really is just the same thing other than the bottled water is filtered from the chlorine etc.
I drink bottled water mostly but the tap water works just fine for making coffee in my Keurig.
There's no need to buy bottled water or special water for the Keurig although I've known some people use bottled water in the Keurig because they prefer it.