Try these GPS coordinates: 37.315101,-76.703861
According to VA DGIF,
Waller Mill Park is located off of Route 645 (Airport Road) between U.S. Route 60 in Williamsburg and exit 234 off I-64. From Route 60, take Airport Road to the east. The park will be on the right after you cross over the upper half of the reservoir. If traveling by way of I-64, take exit 234 to Rochambeau Road. Rochambeau Road will double back to Airport Road. Take Airport Road to the park entrance. The boat ramp and concession stand will at the end of the park entrance road
Also, most facilities like this are an hour before dawn max, otherwise they are sunrise to sunset, including some with gates that close automatically. I have personally not been there before, so it will be new to me as well. If it is locked when I get there, I will simply find a McDonalds if I am way early, or I will just sit and wait for the Parks Officer to come unlock it.
See you there.