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.
At the southwest tip of the Rotterdam harbor. Get on N15, then Europaweg and then Westplaatweg. Get to the beach which is facing the south. Across the bay from the `Autostrand`.