Egypt has a very hot and dry climate that allows you to wear only shorts and a T-shirt most of the year during daytime.
The south, due to its major proximity to the equator, is even warmer than the better-known areas in the north, say Hurghada or Sharm El Sheikh.
Summer season start, in April, after a couple of weeks of strong north winds that indicate the ending of winter.
The sea level starts getting lower and currents run southwards as a general rule. Summertime extends till the end of October or beginning of November, when strong south winds and the only rain of the year break the routine of mild north winds that predominate during the summer.
Also the direction of the currents changes and now eventually run north, while the sea level gets higher from then on.
During the winter, temperatures drop drastically at night, making advisable the use of a jacket or a sweater. Therefore, the best time of the year to be there spans from March through November, when you will he able to dive with a 3 mm full suit but some people prefer diving in this period with 5 mm In July and August some people are diving with shorties,it depends on how every body tolerates the different temperatures.
A double 5 mm will be necessary for the rest of the year.
Following are the temperatures inside and out of the water during the year.
These are average temperatures. Air temperatures are likely to drop at night, up to 8°C below daytime's.

The following temperatures are in Celsius In the North part like Hurghadah and Sharm El Sheikh Water temperatures are 1 to 2 degrees lower

Please check these links for up-to-date weather reports,
Hurghada Weather
Safaga Weather
Quseir Weather
Marsa Alam Weather