Water moccasins are not found in MD. There are venomous copperheads (which can swim) but it's more likely he saw one of the species of water snake. I've seen quite a few snakes both in and out of the water near the C&O Canal & Potomac river. In western Maryland (i.e. beyond the greater DC area) there are Timber Rattlesnakes. Here's a good website with MD's snakes, including photos:
https://dnr.maryland.gov/wildlife/Pa...rSquamata.aspx