First of all, don't forget there are 15 million or so people living in Istanbul. Yes, people sometimes get crazy because they can not eat or drink or smoke for 17 hours. Or, some neighbourhoods are more conservative and they may think it is a disrespect if you eat and drink in public places. But you can be sure there will not be any problem in touristic spots.
Due to laws, there are no additional restrictions for the holy month Ramadan.
In my experience it really depends on where you are. When I lived in Ankara, I found it much more conservative and observant than Istanbul. For example in Ankara I didn't feel comfortable or respectful eating or drinking in public. But now I am in Istanbul during the first week of the holy month and aside from the flocks of tourists, I also see lots of Turks publicly eating, drinking and smoking during daylight hours. But as a general rule, as so many Turks do fast all day, I don't try to shop or hail taxis close to Iftar (the sundown breaking of the fast) as people are hungry and anxious to eat.
Oh and if you happen to be staying in a residential neighborhood away from the tourist centers, don't be alarmed by the local Ramadan drummer who marches through the neighborhood before sunrise alerting people to wake up and eat! I actually find it quite charming...
