In the South Sinai area, I think you can expect water temperatures at the end of May this year to be around 25 degrees Centigrade, plus or minus. At the moment it's 23 in the Sharm area and rising slowly after a cold winter, so it might remain under the yearly average for the time of your visit. Air temperatures you can expect to be around 40 degrees C or more during the day. The Northern Red Sea is more windy than calm at any time of year, and hot desert + cool sea = windy.
I would suggest a 5mm wet suit would be adequate, and if you're doing a 4-dive per day safari then a hood is probably a good idea. I would not laugh at somebody who brought along a semi-dry at that time of year because that's what I will be wearing, and you will only be uncomfortable if you are standing in the sun waiting to jumo. Having said that, a 5mm shorty won't cause you to freeze to death, but I expect you'd be feeling the cold half way through the safari and wind chill can make those night dive exits a bit unpleasant, especially if you are out at sea.
The South will be a little bit warmer, but my knowledge of the area is limited, so I will leave that to others.
Cheers
C.