I'm planning to go to Venice on 26th October.
I've seen some pictures from Venice over winter where everything is completely flooded but I'm not sure which month they have been taken.
Will the weather in the date that I'm planning to go be manageable? And will the tourist attractions actually be open off-season?
There are lots of ‘best time to visit Venice’ guides out there online. Eg https://www.tripsavvy.com/best-time-to-visit-venice-1548173 You can expect fewer tourists, some decent if cooler weather, and some events to look out for (or avoid, depending on your preference). Acqua alta is most likely to happen in November - December http://en.turismovenezia.it/Venezia/High-Water-Information-Centre-7442.html
There is helpful information about this on, of all places, the railway website.
https://www.italiarail.com/flood-warnings-venice
Basically the issue is that as they say
It happens when an unusually high tide pushes water from the Adriatic Sea into the Lagoon helped by southern winds and low atmospheric pressure. The flooding isn’t dangerous and it’s not considered a big deal, as the water usually subsides within a few hours. In the past century it has become more frequent, and now it can happen any time of year.
So you might want to avoid times of the full or new moon when spring tides occur but otherwise it seems you just grin and bear it.
