Even that article doesn't claim there have been any confirmed sightings of water moccasins in MD. Deep Creek Lake sounds esp. dubious given the harsh winters in that far western corner of MD. It's not unreasonable to think that water moccasins could expand their range due to climate change. Southern MD (nowhere near the C&O Canal) would be the most likely part of the state where they'd first appear. But I would also expect that they'd show up first in central VA before they get to Southern MD (defined as the 3 MD counties south of DC wedged between the Potomac River and the Chesapeake Bay.) But I haven't even heard of confirmed sightings in central VA. Lots of folks in the DC area seem to assume that a water snake must be a water moccasin.
BTW, the Washington Post had a cool photo 2 years ago of a copperhead entwined in a chain link fence in East Potomac Park in DC (a short walk from the Jefferson Memorial). The photo was supplied by the National Park Service, which I consider a pretty reliable source.
https://www.washingtonpost.com/local...315_story.html