Don't drink the tap water - it is highly chlorinated. However, with enough time in Egypt, you could potentially grow accustomed to the water. This doesn't mean that the tap water is polluted, it simply means that you are not used to it, and you might get stomach problems. This happened to most people when they visit a country for the first time. They were simply not used to water there. It's a similar situation here. It is advisable to drink Bottled water during your entire holiday. That is why it is strongly advisable that you don't drink the tap water, and use it only for washing and brushing your teeth.