Eastern Egypt and Red Sea, map

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 very hot summers.
Here are the average temperatures.
Marsa Alam - Average temperatures
Min (°C)131416202325262625221815
Max (°C)232427303235353534322725
Min (°F)555761687377797977726459
Max (°F)737581869095959593908177

The sun shines all year around, and it almost never rains.
Here is the average precipitation.
Marsa Alam - Average precipitation

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 on the intensity of the wind, in addition to the individual's characteristics (that is, if you are more or less sensitive to cold). Anyway, it seldom gets cold: on cooler days the minimum temperature drops to around 12 °C (54 °F) and the maximum temperature remains around 18/20 °C (64/68 °F). Occasionally (once every few years), the minimum temperature can drop for a day or two below 10 °C (50 °F).

Summer is very hot and sunny. Typically, the humidity is not very high, and the heat is not as intense as in the interior of Egypt, in addition the wind often blows (though not always), making the heat more bearable. Since we are almost at the Tropic, it's important to drink a lot and to protect yourself from sunburn. Typically, the maximum temperature remains around 35/36 °C (95/97 °F), but when the winds blow from the interior, it can exceed 40 °C (104 °F). The wind from the interior can raise storms of sand and dust and is usually more frequent in spring.

Marsa Alam

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

In winter, the temperature of the sea is a bit cool but still acceptable, in fact it drops to 22 °C (72 °F) in February, while it is very high in summer, since it reaches 29 °C (84 °F) in August.
Here are the average sea temperatures.
Marsa Alam - Sea temperature
Temp (°C)232223232527282928282624
Temp (°F)737273737781828482827975

Best Time

The best times to visit Marsa Alam are from mid-March to mid-April and from mid-October to mid-November. In these periods, the weather is warm or hot (usually, not too hot), 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 spend long periods in the water, maybe doing snorkeling, but you must be prepared to periods of extreme heat, when even lying under a beach umbrella can be tiring.
In winter, it can sometimes get a little cool for swimming and sunbathing, especially for those who are sensitive to cold, although in the central hours of the day you may well sunbathe.