As discussed previously it's the current that kills, not voltage.
Also keep in mind that alternating current is far more deadlier (especially at low frequencies). "Low frequency (50–60 Hz) alternating currents can be more dangerous than similar levels of DC since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, a deadly heart rhythm that must be corrected immediately." -wikipedia
Now there's always the argument that because AC causes muscles to contract so you might actually involuntarily let go of the source of electricity, but in this case where you are having a bath I deem it as irrelevant.
The iPhone uses a ithium-ion battery which all operate with voltages of 3,6-3,7V
DC. Now the human body has a resistance that is dependant on the entry points level of moisturisation. On dry skin it's somewhere in the 100kohm range and with wet skin around 1kohm. When you are in a bath I'd say the skin is pretty wet. Thus we can apply ohm's law and calculate the current (I=U/R). 3,7V/1000ohm = 3,7mA.
According to wikipedia 300-500mA
DC is needed for ventricular fibrillation and this is assuming you are applying the current straight to the chest. In this bath case if you just drop it in a random spot of the tub you have to take into consideration the fact that water in itself is pretty resistive as well making the current that flows through you even smaller.
All that being said it's fairly safe to take a bath with your iPhone.
