Map from Google - Marsa_Alam

The climate of Marsa Alam, on the Egyptian coast of the Red Sea and at the latitude of Luxor, is subtropical desert, with very mild or pleasantly warm winters, and hot summers.
Here are the average temperatures.
Average temperatures - Marsa Alam
Marsa AlamJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)555761687377797977726459
Max (°F)737581869095959593908177

The sun shines all year around, and it almost never rains.
Here is the average precipitation.
Average precipitation - Marsa Alam
Marsa AlamJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (in)00000000000.10.10.4

In winter, from December to February, the temperature is pleasant and the sun shines, but it can be a bit cool for swimming and sunbathing, especially in December and January: it depends on the day and the intensity of the wind, in addition to the individual characteristics (that is, if you are more or less sensitive to cold). Anyway, it seldom gets cold: on cooler days the minimum temperature goes down to around 54 °F and the maximum temperature remains around 64/68 °F. Occasionally (once every few years), night lows can drop for a day or two below 50 °F.

Summer is hot and sunny. Typically the humidity is not very high, and the heat is not as intense as in the inland areas of Egypt, in addition the wind often (but not always) blows, making the heat more bearable. It's important to drink a lot and be protected from sunburn, because we are almost at the Tropic. Typically the maximum temperatures remain around 95/97 °F, but when the winds blow from the interior, the temperature can exceed 105 °F. The wind from the interior can raise sandstorms and dust, and is usually more frequent in spring.

The amount of sunshine in Marsa Alam is excellent: practically the sun always shines, all year round.

Fish in the Red Sea

The temperature of the sea is a bit cool, but still acceptable, in winter, as it goes down to 72 °F in February, while it is very high in summer, since it reaches 84 °F in August.
Here are the average sea temperatures.
Sea temperature - Marsa Alam
Marsa AlamJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°F)737273737781828482827975

When to go

The best times to visit Marsa Alam goes from mid-March to mid-April, and from mid-October to mid-November: in these periods you can enjoy a warm or hot (but not too much) weather, and the sun shines but it's not at the zenith. In summer the heat can be too intense, although typically the wind blows, and being the sea very warm you can escape in the water for long periods, maybe for snorkeling. But you must be prepared to periods of extreme heat, in which even lying under beach umbrella can be tiring.
In winter it can sometimes get a little cool for beach life, especially for those who are sensitive to cold, although at midday you may well sunbathe.