I was in venice about… 5 or 6 years ago actually.
The water isn’t something you’ll want to dive right into. It’s not a place you go swimming after all. BUT if you fall into it? You won’t die. Just don’t drink it or wash with it or anything. It’s as clean as it could possibly be, but I remember then they were making an honest effort to get it cleaned, because they were also thinking of ways to control the flow of the tides since Venice is actually technically sinking into the water too.
So right now it might be clean that you won’t get herpes and pink eye from it. But I wouldn’t recommend doing anything with it.