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.
A29 south from Rotterdam. Then follow signs to Numansdorp. From Molendijk onto Veerweg to the jachthaven and get to the water. The launch is east of the small harbor and its mole.