High season is from mid-June to mid-September with temperatures of around 28°C on average. July and August are hot, especially in the Algarve, the Alentejo and the upper Douro valley (temperatures can be over 40°C). The best time to dive in Azores is from June to November. This is also the time when large pelagic come to the archipelago.
Apart from the best seasons / high season, diving in Portugal is possible all year around with water temperatures ranging from 13 to 24°C (55 to 75°F) with warmer temperatures in the south of the country and the islands of Madeira and Azores. You can comfortably dive in 5 – 7 mm wet or semi dry suits.