Map from Google - Rotterdam

In Rotterdam, the second largest city of the Netherlands, the climate is sub-oceanic, humid and rainy, influenced by the North Sea, but also by the Atlantic Ocean, from which disturbances pass throughout the year, so it has cold (but not freezing) winters, and quite cool summers.
The average temperature in January is 3.5 °C (38.5 °F), while in July it's 17.5 °C (63.5 °F).
Here are the average temperatures.
Average temperatures - Rotterdam
RotterdamJanFebMarAprMayJunJulAugSepOctNovDec
RotterdamJFMAMJJASOND
Min (°C)1134811131311841
Max (°C)671014182022221915107
Min (°F)343437394652555552463934
Max (°F)434550576468727266595045

Precipitation is relatively abundant, about 850 millimetres (33.5 inches) per year, but above all it is common and distributed over the seasons; however, the rainiest season is autumn, the least rainy is spring. Rainfall often occurs in the form of short showers or drizzle.
Here is the average precipitation.
Average precipitation - Rotterdam
RotterdamJanFebMarAprMayJunJulAugSepOctNovDecYear
RotterdamJFMAMJJASONDY
Prec. (mm)706065456065758085908580855
Prec. (in)2.82.42.61.82.42.633.13.33.53.33.133.7
Days1210129910101012121313132

The wind frequently blows, and can be intense, especially from November to March.

Winter, from December to February, is cold: temperatures are on average a few degrees above freezing (0 °C or 32 °F) during the day, while at night they are around freezing; wind and humidity often exacerbate the feeling of cold.
However, different weather situations can occur.
When the westerlies blow, the temperature can be quite mild, and can reach 10/12 °C (50/54 °F), but there can be wind and rain.
On the other hand, when the city is affected by cold air masses from Russia, the temperature drops below freezing, for longer or shorter periods, even during the day. During the most intense and prolonged cold waves, rivers and canals can freeze. Generally the temperature does not drop below -13 °C (9 °F); more rarely, it can drop further, to around -20 °C (-4 °F) in the coldest winters.
Snowfalls in Rotterdam are quite frequent, though usually not abundant, and occur in the form of sleet or light snow, sometimes however snow can accumulate on the ground, and last for many days, if it occurs during cold waves.

Spring, from March to May, is initially cold, with possible snowfalls still in March, and then it gradually becomes milder in April and May, although some cold days, with highs around 10 °C (50 °F), can occur also in May, especially in the first half of the month. However, April and May are, albeit slightly, the least rainy and cloudy months.
Typically, in the second half of May the temperature becomes permanently acceptable, and there can be the first pleasantly warm days, with highs around 25 °C (77 °F) or even above.

Summer, from June to August, is mild, and the flow of Atlantic fronts does not cease even in this season; the wind blows constantly from the sea, and rainfall is also quite frequent. However, there are pleasant periods, in which daytime temperatures are between 20 °C and 25 °C (68 °F and 77 °F), but also cooler periods, in which they remain below 20 °C (68 °F). At night it can be a bit cold, in fact the temperature can drop below 10 °C (50 °F) even in July and August. Sometimes there can be hot days, in which the temperature can reach 28/30 °C (82/86 °F), but this may not occur for a whole month.

Autumn, from September to November, is initially pleasant, ie in the first half of September, then it gradually becomes more cold, dull, wet and windy. The first night frosts usually occur in November, but sometimes also in October.

Rotterdam

The amount of sunshine in Rotterdam is not high: the sun rarely shines from November to February, while it goes a bit better from May to August, when the weather is variable, and between a disturbance and the other the sun can come out.
Sunshine - Rotterdam
RotterdamJanFebMarAprMayJunJulAugSepOctNovDec
RotterdamJFMAMJJASOND
Sun (hours)234677765322

For swimming, the North Sea is very cool or cold even in summer: the temperature reaches 16 °C (61 °F) in July, and 17 °C (63 °F) in August and September.
Here are the average sea temperatures.
Sea temperature - Rotterdam
RotterdamJanFebMarAprMayJunJulAugSepOctNovDec
RotterdamJFMAMJJASOND
Sea (°C)87891113161717151310
Sea (°F)464546485255616363595550

When to go

The best time to visit Rotterdam runs from mid-May to mid-September, being the mildest of the year. July and August are the warmest months; however, it's better to bring a sweatshirt or sweater, a light jacket and an umbrella, for cool and rainy days.