All snakes can swim. It’s not just specialized snakes, like Sea Kraits, that can swim and dive. Water Snakes, Copperheads, Water Moccasins, Garter snakes, Anacondas, Ribbon snakes, Rat snakes, and many more are often found near bodies of water. Even the arboreal snakes of the world like Green Tree Pythons and Mangrove snakes are competent swimmers.
The museum grounds are home to a number of resident snake species; including the only venomous snake species in Durham, the Copperhead. While I didn’t see a copperhead on this occasion, I did see the Northern Water Snake below who just happened to be swimming by when I was on the boardwalk.