Mapquest seems to disregard ferries. Although they might not be the fastest way to get somewhere, they are the most fun way to get there. I am only mentioning them to give you other options. My 2 year old atlas shows several ferries to the lower Eastern Shore.
Take Rt. 360 all the way to the Reedville/Smith Point area at the mouth of the Potomac. The Reedville ferry take you to Tangier Island. From there, another ferry takes you to Onancock, VA, near Rt. 13N. T he Smith Point ferry takes you to Smith Island (?) where another ferry takes you to Crisfield, MD. Take Rt. 413 to 13N.
If you want to travel through MD before you get to the ferry (way longer), you can take I-95 to exit 104, Rt 207 to Bowling green. From Bowling Green, take 310 across the Potomac to the Rt. 234 junction . Go down to Rt. 5 and take that to Point Lookout. The ferry from there goes to Smith Island(?) and Crisfield. I have put (?) after Smith Island because the atlas only names one of the 3-4 islands in the group.
I have no schedule for these ferries, so I don't know when they run, or how often. Personally, if I was going to the MD beaches, I would take the southern route though Norfolk and the northern route for the DE beaches; forget about the ferries