As "ajduplessis" has mentioned each diver experiences temperature differently as such it's very hard to answer your question. I, for example, wear a full 5mm In Thailand and am fine, tried a 3mm and was cold. Locally I wear a 2piece 7mm FJ with hood down to around 62 Im fine anything below that Im out of the water. It's better to be too warm than too cold in a wet suit, you can always open the neck and flush. If cold there is not a lot you can do to then get warm. Whatever you go with I'd recommend a full suit for sun protection and the occasional stinging creature. Although I haven't dived in Egypt my understanding is that it can get cool depending where and when you dive. Im not sure Egypt would be classed as "tropical" perhaps more "temperate". Good luck.