Experts and BHG readers answer.
Water grass in yard
First, I'm going to assume that you mean crabgrass. There are other grassy weeds that people sometimes call "water grass" but usually they mean crabgrass so I'll go with that.
There are a few chemicals that kill crabgrass on lawns without killing the lawn as well. Find a garden center and ask for a POST EMERGENT herbicide for crabgrass in lawns. Some of these can discolor or stunt lawn grasses, FYI, but it's temporary.
The better solution is to apply a pre emergent, or weed preventer, in spring before crabgrass germinates. In your area, late April is a good time to apply it. It's too late to do it this year, obviously, but next spring, put some down and you won't have much of a crabgrass problem.