Read the many postings in online travel forums about the dire state of drinking water in St Petersburg, and you're likely to stick with bottled water for the duration of your stay. There is no doubt at all that for many years the city's water supply was completely off limits due to the presence of a water-borne parasite which causes giardiasis, a particularly unpleasant stomach infection. Eating food which had been made from this water such as ice cream, or fruit or vegetables which had been washed in it, also exposed the individual to potential transmission of the parasite.
Today a large proportion of the city water infrastructure has been upgraded, the water is free of giardiasis, and the only remaining concern is the state of the some of the aging water pipes the city's water supply passes through. It should be perfectly safe to drink the water from the tap at most hotels and many apartments in the centre. We drink it, largely for environmental reasons (imagine how much plastic is thrown away every day from bottled water), although many locals also avoid it where possible.
You may occasionally need to run the tap before drinking the water as the water can be brackish or slightly coloured. However brushing your teeth should be fine. Even though the authorities claim the water is perfectly safe today, use your judgement. We recommend that anyone susceptible to stomach trouble use bottled water, which can be found in local and international brands and various sizes with ease.