Map from Google - Faro

Algarve, the southernmost region of Portugal, with Faro as the capital, has a Mediterranean climate, mild and sunny. There are beautiful beaches, and in the ocean you can also surf.
Here are the average temperatures.
Average temperatures - Faro
FaroJanFebMarAprMayJunJulAugSepOctNovDec
FaroJFMAMJJASOND
Min (°C)91011131418202019161310
Max (°C)151618202225282826221916
Min (°F)485052555764686866615550
Max (°F)596164687277828279726661

Precipitation is not abundant, since it's lower than 500 millimetres (20 inches) per year, and follows a Mediterranean pattern, being more abundant from October to March, rarer in April and May, and almost absent in summer.
Here is the average precipitation.
Average precipitation - Faro
FaroJanFebMarAprMayJunJulAugSepOctNovDecYear
FaroJFMAMJJASONDY
Prec. (mm)705070302051120506565447
Prec. (in)2.822.81.20.80.2000.822.62.617.6
Days76853100247750

The wind blows frequently: in the cold half-year it's related to Atlantic depressions, while in summer there are sea breezes in the afternoon.

Winter, from December to February, is mild, and is characterized by sunny periods, alternating with periods of bad weather, with wind and rain. Very cold days are rare: only in the coldest nights the temperature drops to freezing (0 °C or 32 °F), but even in these cases, during the day it typically rises above 10 °C (50 °F).

Spring, from March to May, is mild and sunny. In March and the first half of April, there may still be a bit of instability, with some rain.

Summer, from June to mid-September, is warm and sunny. Some days may have an "Atlantic" weather, with highs around 25/27 °C (77/81 °F), while at other times there may be hot days, with highs around 35 °C (95 °F); however, sea breezes temper the heat.

Autumn, from September to November, is initially warm and sunny. In September, the first disturbances may occur, but are still rare. Even in October there are many warm and sunny days, especially in the first half of the month. In November, it's now full autumn, with a mild weather, but also often cloudy and rainy.

The amount of sunshine in Faro and Algarve is excellent in late spring and summer, from May to August, while it is acceptable in autumn and winter, when in addition to sunny periods, cloudy and rainy days are possible.

Beach in Algarve

For swimming, the waters of the Atlantic Ocean are never very warm, and are still a bit cold in June, when their temperature is about 19 °C (66 °F), then it goes up to 20 °C (68 °F) in July and 21 °C (70 °F) in August and September, and then returns to 20 °C (68 °F) in October.
Here are the average sea temperatures.
Sea temperature - Faro
FaroJanFebMarAprMayJunJulAugSepOctNovDec
FaroJFMAMJJASOND
Sea (°C)161516161719202121201817
Sea (°F)615961616366687070686463

When to go

The best time to visit the cities of Algarve, for walks and hikes, is the spring, in April and May.
For the sun and the sea the best time is instead the summer, from June to mid-September. It's better to bring a sweatshirt for cool and windy evenings, especially in June and September, but for safety even in July and August, especially if you are sensitive to cold.