You can absolutely drink the water in Venice and any of its fountains with absolute safety. It doesn’t taste very good because it is very minerallly, having gone through the same pipes for over a thousand years. But it will never make you sick, unlike the water in thousands of cities in the USA:
https://www.scientificamerican.com/a...beyond-flints/

in restaurants in italy it is recommended that you drink bottled water with a meal instead of perfectly safe tap, because the tap tastes bad. It would be like having a great dinner and instead of pairing it with a great wine, pairing it with a cheap beer.

The water in Venice is 100% safe, but it doesn’t taste very good.