There are signs, but since it only really floods severely like that once or twice a month it’s easy to forget. Some signs have flashing lights on days the tide will be super high.
About the water quality, it can vary. Sometimes we hear that water quality is way up and old species like water voles are coming back. Then we’ll hear that they’ve released thousands of gallons of raw sewage into the river, cause they still do that in an emergency.
I avoid river water in general and certainly would be taking a hot shower if I was in it like those people.