South Mountain Inn is not a bad stop off for some munchies, and right on the AT. It's a bit pricey. There's not much else along the trail, except a few vending machines at Gathland.
Keep in mind the AT through MD is on State Park Lands and you are required to stay at shelters/designated spots.
Water should be plentiful near the shelters. Raven Rock and Ed Garvey shelters water source is quite a bit away. 1/4 mile in fact. Raven Rock isn't too bad. It's a heck of a climb down from Ed Garvey though. I did it once, and won't do that again.
There is a water spigot at Washington Monument and Gathland and free water/bathrooms/showers at Dalhgren Backpack Campsite.
The .2 mile loop side trail at High rock, near the PA border, is worth it! Great views from Raven Rock, Black Rock, Annapolis Rocks, and the Washington Monument too.
Enjoy your hike!!