Spring: air 14°C, water 12°C. Summer: air 20°C, water 16°C. Fall: air 12°C, water 12°C. Winter: air 4°C, water 8°C. Season is year round in principal, but May to October is the best. Wetsuit is necessary, especially in spring and autumn. Winter can be really cold. In summer a shorty is sufficient.
The Dutch islands have often more wind then on the mainland, so it's often better than somewhere else in the country. You can vary on the spots you go to, depending on the wind. Spring and fall are the best for good winds. Winds are based on high and low pressure areas. Wind in spring and autumn usually 3-4 times a week. In summer it's less.
Drive to Friesland in the Netherlands, drive past Leeuwarden to Holwerd, take the boat here. The spots south are easily reached by car/bike. The north-spots are only reached by a car that has license to ride on the beach. Otherwise you can take the bike to go there.
