Marsa Matrouh, where it is

In Marsa Matrouh, on the northern Egyptian coast, the climate is Mediterranean, with mild and relatively rainy winters, and hot and sunny summers.
Here are the average temperatures.
Marsa Matruh - Average temperatures
Min (°C)9911131519212120171410
Max (°C)181920232528293029262319
Min (°F)484852555966707068635750
Max (°F)646668737782848684797366

Rainfall is scarce, since it amounts to about 150 millimetres (6 inches) per year, therefore it's at desert levels; it still follows the Mediterranean pattern, that is, it occurs from October to April, and is very scarce or absent in late spring and summer.
Here is the average precipitation.
Marsa Matruh - Average precipitation

Winter, from mid-December to February, is mild: the maximum temperatures fluctuate generally about 18/19 °C (64/66 °F). There are long periods of sunshine, alternating with periods of a few days with a bad weather and rain, generally not abundant. It rarely gets cold, and on colder days the temperature can go down to around 5 °C (41 °F) at night, and stay about 10/13 °C (50/55 °F) during the day.

Spring, from March to May, is pleasantly warm, with a lot of sunshine, and the rain becoming progressively rarer. In April, it can sometimes be a bit cold at night, with lows around 10 °C (50 °F). At other times, the hot wind from the desert can bring very high temperatures, especially in April and May, with peaks of 40 °C (104 °F).

Summer, from June to September, is hot and muggy, but with lively breezes that temper the heat. The maximum temperatures in July and August are generally about 28/30 °C (82/86 °F). Heat waves from the Sahara are rare, however, they are more likely in June.

Autumn, from October to mid-December, is pleasantly warm, often sunny, but also with some chance of the passage of depressions, and as a result of rainy periods, especially in November.

Marsa Matruh

The amount of sunshine in Marsa Matrouh is very good in summer, when clear skies prevail, and it is also good in the other seasons, when, however, cloudy days can sometimes occur.
Marsa Matruh - Sunshine

The temperature of the sea is a bit cold in winter, it is still very cool in May, when it's about 20 °C (68 °F), it becomes acceptable in June, when it reaches 23 °C (73 °F), and then it becomes warm, about 25/26 °C (77/79 °F), from July to October.
Here are the average sea temperatures.
Marsa Matruh - Sea temperature
Temp (°C)181717182023252626252220
Temp (°F)646363646873777979777268

When to go

The best time to go to Marsa Matrouh, for swimming and sunbathing, runs from mid-May to mid-October. In April it often gets a little cold for sunbathing, except during the warmest hours, and the sea is still cold. Until mid-June, sometimes it can be a bit cool in the evening, so those who are sensitive to cold should bring a sweatshirt. You can bring a light sweatshirt, for windy evenings, also in mid-summer.