The climate in Greece is typical of the Mediterranean climate with two real seasons - a warm and dry summer, and a mild and wet winter. Summer is a long season with temperatures climbing to a pleasant degree as early as late February. However, March and even April can be somewhat unpredictable months with the occasional rain shower and windy conditions. On the plus side though, at this time of year the landscapes are decorated with wild flowers.
The shoulder months between May and June still offer lovely weather conditions but without the crowds. It's warm on the islands and the waters are nice enough for a swim. Peak tourist season doesn't start until mid-June and lasts until early September. During this time the islands are buzzing with people, both travellers and locals alike. If you're visiting during this time it's best to book hotels, flights and local ferries as far as in advance as possible to avoid disappointment and secure better prices.
Between September and mid-October the soft light and delicate colours are particularly suited to photography. The sea water retains the heat of the mid-summer months so swimming is definitely still on the cards.