Water is considered safe to drink throughout the Czech Republic. It is not the norm to serve tap water in restaurants; usually your request for glass of tap water in the restaurant will be politely refused or will lead to a long discussion with the waiter!
The rate of violent crime in Prague is generally low, although pick-pocketing and petty theft can occur. Be especially alert at the main tourist places such as the Old Town Square (specially while you are waiting in front of Town hall for the Orloj to start), Wenceslas Square and whilst crossing Charles Bridge.
Emergency numbers: You can dial the general emergency number 112 or the specific numbers for the different emergency services: 150 for fire brigade, 155 for ambulance or 158 for police.