Late spring (May and June) and early autumn (September and October) bring warm, sunny days, a swimmable sea and easier access to the Calanques, with lighter crowds than midsummer. July and August are hot and very busy, and high fire risk can close parts of the Calanques, while winter is mild but windy, with the cold mistral sweeping through on clear days.
Pickpocketing and bag-snatching are the main risk, concentrated around the Old Port, La Canebiere, the Noailles market and on crowded trams and airport buses. Keep bags zipped and in front, avoid back pockets, and stay alert in tourist crowds and on packed transport.
Marseille's reputation for crime is largely tied to the northern housing estates (quartiers nord), which hold little of interest for visitors and are best avoided, especially after dark. The central tourist areas you will actually visit are generally fine with normal city caution.
Summer sun is strong and the Calanques offer little shade and no water, so carry plenty and start early. On high fire-risk days in summer, the authorities restrict or close access to the massif, so always check the day's rules before hiking.