However, to choose the correct diving suit, I would like to know the temperatures that can be expected at a depth of about 30 meters (100 ft) at the beginning of October.
A good estimation is to deduct 5 degrees Celsius from surface temperature to get temperature at 20m - 30m depth. (This is based on my experience over thousands of dives, but most of the diving is in temperate and colder waters)
There are several factors that can influence the difference between the temperature at surface and at depth.
I would say contact a few dive shops in the area you wish to dive and ask them from their practical experience what temperatures you can expect at depth. Then choose good thermal protection that will work on shallow and deep dives.
I recently dived at Dahab for the fourth time, just along the coast from Sharm, in October 2015 at depths of 20-30m. I don't tend to feel cold when diving at that depth any time of year in the Red Sea. I was completely fine with my usual 2.5mm shorty and if I remember correctly the water temperature was around 25 degrees at that depth.
Now, a few years after asking the question, I have gathered some data over the course of 4 trips between 2015 and 2019, all of them in October:
In 3 of these years, it was 27°C to 28°C at around 30 m (± 5 m). All of these trips were before October 20.
In one year (2017), it was only 25°C at that depth (after October 20).