Sharm el-Sheikh has a hot, dry climate with two broad seasons. The sea is warm and swimmable year-round — the question is mainly about air temperature and tourist crowds.
Winter (November to March)
The most pleasant time to visit. Daytime temperatures range from 22 to 27°C, nights can drop to 13–15°C. Diving visibility is at its best in November and March, which draws large numbers of divers during those months — book ahead if your dates coincide. Rain is rare but possible: occasional short, heavy bursts, perhaps once a month.
Summer (April to October)
Daytime highs reach 35–45°C, making sightseeing and trekking uncomfortable in the hottest months (July–August). This is paradoxically peak tourist season, particularly for European package holidaymakers, so flights and hotels are more expensive and beaches more crowded. The sea stays beautifully warm. Watch out for the Khamsin, a hot desert wind from the Sahara that arrives in spring and can kick up sandstorms lasting several days.
In short: If you are mainly here to dive or trek into the desert, come in winter. If you are coming primarily for the beach and the water, any time works — but expect to pay more and share the beach with more people in summer.