Water temperature varying from 14-23°C. 5mm wetsuit mandatory in winter. Air temperature going from 6-30°C. Predominant wind in winter: southern with not too bad weather. Northern wind with cold weather and rain.
Stronger wind from September to March, from the South. If there's a `low` coming, it will probably blow. During summer you can find 12 knots many afternoons. The hotter the day, the stronger the wind. Predominant wind in winter: southern from 10-40 knots. Northern, from 10-25 knots. Summer: thermic wind from the north, never exceeding 20 knots.
During the summer, you will find that in a 20 km. radius one or more towns will be celebrating their local holidays. During winter the action gets restricted to Gernika and Bilbao, and only on weekends. Many places to drink a beer with your friends, though.