The high season in Portugal that lasts from May and to the end of September offers very warm weather (average around 27 C) and smaller waves. Nevertheless, at the places like Ericeira, situated close to the coast, it is never too hot and you can expect some wind, which can get a bit chilly in the evening. It is nice to have some sweaters, trousers and sneakers to use in the evening and at night.
From October till March you can still get some, you still have a lot of sunny days, but this is the time when the winds are getting stronger and the waves bigger. You should expect some rain as well during these months. The average temperature would be around 15 C, but it can be higher up to 22 C in October and March and can drop a bit (10-12 C) in December-February.
The temperature of the water in summer can be up to 20 C, but you still need to surf with a wetsuit. In winter it can drop till 12 C, but you still feel very comfortable if you are using a good wetsuit.