Why does my kitchen faucet have low pressure after the water heater was replaced?
9 times out of 10, like Tester mentions, this is going to be the aerator. Unscrew it, run some water without it, and clean it before screwing it back on. There's all kinds of stuff that gets knocks around in the pipes when water is shutoff and things are added or replaced in the plumbing system, and the best place for them to end up is the kitchen sink aerator since it's so easy to fix.