It is recommended you drink 1- 2 litres of good quality water per day during pregnancy.
While there are filtration and chemical treatment of tap water, there is an increasing concern in the community about its quality. There is no guarantee of the quality of water in bought bottled water.
The safest option is filtered water. But check the quality of the filtration system, its guarantee about what it removes and its certification. Also, the filtration system should be able to leave important minerals such as calcium and also fluoride in the drinking water.