The water quality of the Grand Canal has been a subject of concern in recent years. While efforts have been made to improve the canal's environmental conditions, factors such as pollution and the impact of tides can affect water quality. The quality is a lot better than it once was but don't do something crazy like go for a swim. You wouldn't believe how often I am asked about that. If you want to swim go to one of these best beaches closest to Venice.  If you've some other crazy question about Italy, you might well find I've already answered it here.