Air: May to August about 15-30°C, September to April: 0-15°C. Water: May to August fine (a shorty will do), from September to April you need a wetsuit, in December/January it can get cold (around 0°C). January to August unpredictable, can be anything. September to December usually cloudy/rainy weather though in this period, especially when there's wind.
From Amsterdam take N203 north and follow signs to Uitgeest. Here get to the lake and the swimming area. From here about 400 m east is a quiet launch beach.