Dahab temperature differs from 21 C in winter season to 39 C during the summer meaning that even in cold temperatures you can dive, snorkel and swimming.
water temperature is varying from 20 C in wintertime to around 28 C during the summer and there is hardly ever any rainfall, since the Sinai features a desert weather.
Summer time is hot and dry and you'll hardly see any clouds, but the low moisture together with gentle Dahab breeze make the heat bearable. Throughout the summer it is important to drink plenty of liquid in order to avoid dehydration.
Winter months months between December & March have actually a tremendously comfortbale weather, although Dahab heat through the night can drop quite a bit and it might be windy, it is therefore important to bring some hotter clothing even if you may well not require all of them during the day.