We are having a particular week here due to the high tide causing the so called phenomenon "acqua alta" (hight tide), so the city floods and water is pretty much everywhere, a big surprise for many tourists here, friends and people around the world that are clearly not familiar with this and they are comprehensibly surprised and sometimes worried! It's obviously strange and bit scaring to arrive in Venice and be involved in this situation:
Ah for most of the time it rains! Pictures about this were all over the internet, newspapers all over the world spoke about it, friends wrote to ask if I was ok, and safely from my couch I was reassuring everyone. This phenomenon happens every year, especially between the end of October and November and then in winter, sometimes early spring, always according with the moon phases and the warm wind Scirocco. Actually this year the situation was exceptional, with a weather alert also because it was pouring down and because of the strong wind, so we had the 4th "acqua alta" ever, 1,56cm for the first time after 10 years. So the concern, for someone who doesn't live here and sees pictures on the internet or on the news, is more than comprehensible.
Anyway there's nothing dangerous if you watch your step and take care about where you are walking.... (thank s to "Venezia non è Disneyland" for the link), but yes it's annoying if you work here and most of all for the ones running an activity.
The ones with they shops spent the worst 24 hours possible, pumping the water off the shop with pumps and buckets but also cleaning everything pretty carefully the day after, because no, this water is definitely not clean! (sorry for the your heroes diving in St. Mark's square!). For tourists well....it depends, if as a tourist you are already here, maybe couple of days before you will have time be prepared, buying rubber boots or covers for your shoes (if you take a proper tour you will also learn the meaning of this blazoned brand) and ready to enjoy one of the most unique experiences ever! It you want the premium experience buy the high rubber boots that cover the entire leg so you will be able to walk in the most amazing square/swimming pool in the world.
If you arrive in town the same day instead, be prepared for an unexpected walk in the water carrying you luggage over your head and one of the most annoying experience ever tried!
October is an amazing month to visit Europe and Venice as well, but the end of the month is not always great, obviously it depends about your aims and expectations, so remember to check the forecast (unless you booked six months earlier!), ask, inform yourself and download the app hi!tide (apple - android), nothing won't scare you!
Just remember, don't stop to ask for directions to locals while busy throwing the water off their shops, they won't be polite and that water could arrive really close to you!