hey there!
it is indeed hard to get answers when it comes to water-temperatures in the red sea. just came back from a trip in march (south st. johns caves- and zabargad-area) and going there again in 3rd week of march. Where are you going and which company did you choose for the trip?
In March it was 23-26°, which was for me fine with a hooded vest... When I was there in May last year it was roughly the same, maybe 1-2° warmer. Since most people on the boat got colder than I they were wearing from 3mm shorty up to 7mm semi-dry. What I would definitely recommend is the hood since you get cold quite quickly on the zodiac and you can also quickly hurt your ears and get an infection.
Personally I am used to dive in tropical waters like the caribbean with 27-29° C and dive there only with a rashguard and my shorts. In egypt I dive with maximal a 3mm full-wetsuit. This time I tried it again with only my rashguard, the hooded vest (vest 3mm, hood 5mm) and I was fine most of the time. On the 3rd or 4th dive i eventually took my 3mm just because you lose so much energy during the day and get cold quicker during the dive.
Hope that helps
Thomas