Tap water is generally safe, but bottled water is readily available and better to drink.
Restaurant food is generally safe to eat. Avoid dishes made with ground meat. Although traditionally Greece has had a low crime rate, in recent years this has changed somewhat, with theft on the increase, particularly in crowded places like urban areas and holiday resorts. Don't be paranoid, as many people are scrupulously honest, but for your own peace of mind keep your valuables under lock and key at all times. For emergency police reaction, dial 100.