Growing up in southern pg county (rural) we did call water snakes "mocassins". I wasn't so worried about whether that was accurate or not and you couldn't just Google it.
We were very familiar with copperheads. I had one cross the tow path in front of me about 5 miles north of Brunswick while I was out running a couple years ago. I waited while he stared at me before finally he headed into the grass. I always considered them kind of aggressive and have seen them chase people before. From that article it sounds like mocassins are a lot more aggressive, which is not how I remember the swimming snakes we had around where I grew up.