Egyptian Red Sea is actually the northern Red Sea, also in its most sothern parts. April-May is the "change" period and water temperatures can range from 22 to 25 degrees celcius, depending on when in this period and where -- roughly a degree or two warmer in St Johns compared to Brothers (it's quite a bit further south). A 7 mm would be advised for 3 dives/day or more on a liveaboard. A 5 mm would suffice for resort diving. August 28 degrees and 3-5 mm is good.