There is a shortage of fresh water, let alone potable water in Egypt. Egyptian tap water is highly chlorinated (not poisonous) and this can upset the microbes in your stomach, causing diarrhoea. Because it is highly chlorinated it is no problem to brush yur teeth with it or if you swallow small amounts whilst in the shower.
As a general rule as a traveler you should always use bottled water for drinking. If the bottled water doesn't taste right, even if it was unsealed in front of you, do not drink it.
The western style ‘sit down’ toilet is prevalent in Egypt, but with additions installed to wash yourself with water after a visit, like the Egyptians do:
-A metal 'eye' or tube inside the toilet bowel which takes aim at the user’s rectum. A tap to the side of the seat releases a stream of water of varying ferocity from the tube.
- A small showerhead on a water hose hanging to one side of the toilet which you can aim and compress a trigger to release the water.
You can use your left hand (hence Egyptians always eat and shake with the right) for the purpose of cleaning yourself after a visit.
Inside resorts you can usually throw your toilet paper in the toilet and flush but outside the resorts the Egyptian plumbing can not take the toilet paper because the sewer pipes will get clogged. Instead all paper is to be placed into the bin provided or do like the Egyptians and wash with water after a visit.
In desert and dry areas you usually find squat toilets. After you relieved yourself, you wash yourself. In contrary to what Westerners might believe: Squat toilets are very clean and in the natural squat position our bodies are aligned in the best position to relieve ourselves.
Some eco friendly accommodations in water scarce areas use composting toilets. You use (wet) paper to clean yourself (or you can choose to wash yourself with water down there if you like to). These toilets are emptied periodically and the content will be burried and used to fertilize the land.
If you are on the road and need to use a toilet, you are advised to bring paper (or wet tissues), hand sanitizer, a strong stomach and small change for baksheesh with you.