The American Coot (Felica americana) is a fairly recognizable water bird more popular in the northern midwest. Maryland is not a breeding range for the Coot, typically, but it can be found along the Maryland shore year-round.
The Coot has a very dark black head and an obvious red eye. The white bill is also decorated with a red ring toward the tip. In the water it will look almost identical to a duck in its form and swim style.
This one was spotted in Canal Wildlife Management Area.
For whatever reason, Canal can be a great place to spot some rare water breeds. I saw Ruddy Ducks there once and have seen Heron and Egret on several trips.