I know this is a long dormant thread and the specific post is 10 years old, but I'm pretty sure there are no "water moccasin"/"cottonmouth" snakes in Maryland. What was most likely encountered was a Northern Water Snake - (non-venomous). https://dnr.maryland.gov/wildlife/Pages/plants_wildlife/herps/Squamata.aspx?SnakeName=Common Watersnake
The venomous snakes found in Maryland include the Timber Rattlesnake and Copperhead.
https://dnr.maryland.gov/wildlife/Pages/plants_wildlife/herps/Fieldguide_OrderSquamata.aspx