I was solo in Florence in 2009 and for the most part found it quite easy and comfortable to eat well (enough) as a solo diner. However, in the 3 days I spent in Venice that year, I had mediocre and a really terrible experience in a restaurant, so I come for advice. The terrible experience was in a place near to the Rialto Bridge that a young local girl recommended to me. It was filled with locals and initially seemed quite pleasant--definitely non-touristy. I was the only woman alone (I am middle-aged for what it's worth). I was placed at what was really a tray, not a table, they took my order, and then I was ignored for 40 minutes. Finally I actually stood up and stared at the host over the heads of all those people. Eventually I got my food but it was a miserable experience. I will say that in the US, solo women diners are often very well looked after. Anyway, I don't remember the name of the place. It was somewhere on the Grand Canal right near the Rialto Bridge.
Now that I've unburdened myself, I hope you Venice experts can direct me to places where I will be comfortable eating alone and can eat well. I tend to eat mostly fish for health reasons...but am open to all your ideas. I will be staying in the middle of the Castello area--maybe 12 minutes walk to San Marco if that can guide you.
Grazie a tutti!