In the US, most snakes (and most that swim) are non poisonous. A lot of snakes swim and the northern water snake is fairly prevalent in Chesapeake creeks and tributaries. They have a variety of markings and some look very much like copperheads, the only venomous snake in MD. They also hang around rip rap, just like copperheads so not fun to be cleaning the bottom and look up to see a snake swimming along. In our area, copperheads are around but not aggressive and probably over reported due to the number of water snakes.
You can find Timber Rattlesnakes in western Maryland. They are somewhat uncommon, but my son came across one last year while out hiking.