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 goes down 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. It's important to drink a lot and to be protected from sunburn, because we are almost at the Tropic. 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

The temperature of the sea is a bit cool, but still acceptable, in winter, as it goes down 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

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 in the central hours of the day you may well sunbathe.