The fastest way, depending on your arrival time, would be to fly Southwest from BWI-ORF and rent a car. ORF is an hour or less drive from Williamsburg.

Otherwise, BWI is about 3 1/2 hours drive time from Williamsburg with no traffic...could be as much as 5+ hours though if the interstates are backed up. The Beltway and I-95 in Northern Virginia are pretty bad traffic bottlenecks, and even I-64 east of Richmond can be congested at times due to it only having two lanes in each direction.