Chesapeake Bay is a huge estuary. Completely salt water up to the Bay bridge, then it magically goes very brackish to fresher water immediately above it. Weirdest thing. We were just south of the bridge and noticed all the nettles around us. Got north of the bridge and they were all gone.
Water snakes are everywhere lol....got one that sometimes suns itself on my pier.
Draw a line from Texas across to and around Florida up to North Carolina and that's gator territory. Oh yeah, don't forget, Florida has the American crocodile too lol.
We have eastern diamondback rattlers, cottonmouths and copperheads.
What you haven't mentioned are the SHARKS lol. Pretty much any shark can go into any salt water, but bull sharks....they can travel into fresh water for periods of time. Bulls have made it up the Mississippi river as far as Illinois, though that tends to be a lot rarer these days with the sheer number of locks and dams that exist now.