There are plenty of good options but these are two I have personal experience with that I would recommend:
Herrington Harbor North is probably the largest boatyard on the Chesapeake Bay, it is on the western shore, about 30 minutes (by road) south of Annapolis and is well supported by a number of mechanical vendors. It is not cheap but well run, very professional and easy to access. If you download the service request form from their website, you will see a list of vendors that work out of this yard, I would check availability with the diesel vendor prior to going in this direction.
Another good option on the Western Shore of the Chesapeake Bay is Washburn's Boatyard (at Calvert Marina) in Solomons Md. Gary Gateau (Drum Point Marine) is a well regarded Cummins mechanic who works out of whatever boatyard in Solomons that is convenient for you. Solomons is further from an an airport or mass transportation than other areas but is a beautiful port you would enjoy spending some time in
I'm sure there are plenty of other options, but these Are two to start with. Definitely make sure the mechanic can support your schedule before making any commitment.
