Biarritz: Annual Weather Averages. August is the hottest month in Biarritz with an average temperature of 21°C (69°F) and the coldest is January at 8°C (46°F) with the most daily sunshine hours at 14 in July.The wettest month is November with an average of 150mm of rain.The best month to swim in the sea is in August when the average sea temperature is 22°C (72°F).he wettest month is November with an average of 150mm of rain. The best month to swim in the sea is in August when the average sea temperature is 22°C (72°F).