The rivers here in VA are loaded with various creatures,...big and small (alive and dead); muskrats, otters, snakes, all kind of bugs, etc.
There are some parts of the river systems that could be 'marsh-like' but only when the river is low.
Most of the time 'they' all avoid you and are seen moving in the opposite direction...of course sometimes they panic and move towards YOU...
I usually end up 'smacking' the snakes with my fly rod or my wading stick.
For the most part they turn and move off. If a snake is going to bite you waders won't help much...and yes snakes can bite underwater!
I only had one bad issue because of a 'fresh' dead cow I found floating in the river, while wet-wading. It only became an issue on the way back ...down-stream... I ended up punching a hole in my leg on a rock. Of course it got infected (10 days on anitbiotic and it was gone) I never had that occur while wet wading so can only assume it was from the dead cow.
There is always the 'creep' factor when wet-wading in 'marshy-still-water.'
Of course when your fly gets slammed it goes away fast!