It depends on what you are looking for.
There are at least two excellent boats in Ocean City Maryland. jrmiah mentioned
Dive Ocean City | Ocean City Diving | Maryland SCUBA Charters | Dive OC | Wreck Diving, charter boat, Atlantic, Mid-Atlantic, dive boat, OC Diver but there is also the OC Diver at
OCDIVER Subheading Goes Here They both go from Ocean City, Maryland but mostly visit different wrecks. The 2010 schedule for the OC Diver isn't up yet, but you can look at last years schedule to get an idea of where Ted goes. If you know the type of diving you want to do, you can't go wrong with either. Visibility out of Ocean City is typically around 20 feet, but it can be much worse or much better.
There is also a dive boat that just started with Off the Wall Scuba. I think they are going to operate in the Chesapeake Bay. There are some interesting wrecks in the bay, but visibility can sometimes be a problem. On more than one occasion, I've experienced black water dives where there was zero visibility. But if you want to see a uboat close to home, or Americas first battleship (USS Texas, renamed the San Marcos before being sunk as a target ship by Billy Mitchell) those are chances you take.
As for dive shops, I would use whatever is closest to you. I am partial to the Brass Anchor, but there are several others I like as well.
Finally, there are a bunch of quarries around including Hydes, Millbrook, Bainbridge, willow springs, Lake Rawlings and a bunch of others. Lake Rawlings is the best in my opinion, but they are all quarries and I only go to them to socialize or practice.
If you are interested in Ocean City, let me know and we can ride together.