Map from Google - Amsterdam

In Amsterdam, capital of the Netherlands, the climate is sub-oceanic, humid and rainy, influenced by the North Sea but also by the Atlantic Ocean, which is not far, so it has cold (but not freezing) winters, and quite cool summers.
The average temperature in January and February is 3.5 °C (38.5 °F), while in July it's 17 °C (62.5 °F).
Here are the average temperatures.
Average temperatures - Amsterdam
AmsterdamJanFebMarAprMayJunJulAugSepOctNovDec
AmsterdamJFMAMJJASOND
Min (°C)0024810121210741
Max (°C)5581216192121181496
Min (°F)323236394650545450453934
Max (°F)414146546166707064574843

Precipitation is relatively abundant, about 840 millimetres (33 inches) per year, but above all it is common and distributed over the seasons; however the rainiest season is autumn, the least rainy season is spring. Rainfall often occurs in the form of short showers, or drizzle. On average, there are 12/13 days with rain per month in autumn and early winter, and about 9/10 days in late winter and spring.
Here is the average precipitation.
Average precipitation - Amsterdam
AmsterdamJanFebMarAprMayJunJulAugSepOctNovDecYear
AmsterdamJFMAMJJASONDY
Prec. (mm)654560505060707075809080795
Prec. (in)2.61.82.4222.42.82.833.13.53.131.3
Days1310910910111111121212130

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

Amsterdam

Winter, from December to February, is cold: the daytime temperatures are on average a few degrees above freezing (0 °C or 32 °F), and at night they hover around freezing; wind and humidity often exacerbate the feeling of cold.
However, different weather situations may occur.
When the westerlies blow, the temperatures can be quite mild, and can reach 10/12 °C (50/54 °F).
On the contrary, when the city is reached 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, the city's canals can freeze. Generally the temperature does not drop below -13 °C (9 °F), as happened in December 2010, or -14 °C (7 °F), as happened in January 1985, January 1987 and January 2010; more rarely, it can drop below these values, as happened in February 1956 (when it dropped to -20 °C or -4 °F) and in February 2012 (-19 °C or -2 °F).
Snowfalls in the Dutch capital are quite frequent, though usually not abundant, and occur in the form of sleet or light snow, however sometimes the 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 the relatively less rainy and cloudy months.
Typically, in the second half of May the temperatures become 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 is characterized by pleasant periods, in which the daytime temperatures are between 20 and 25 °C (68 and 77 °F), alternating with cooler periods, in which even the daytime temperatures remain below 20 °C (68 °F). At night it can be a bit cold, and lows 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.
The wind blows constantly from the sea, and even rainfall is quite frequent.

Autumn, from September to November, is initially pleasant, 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.

The amount of sunshine in Amsterdam is not high: the sun rarely shines from October 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. There are on average 7 hours of sunshine per day in May and June, and 6 in July and August.

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 - Amsterdam
AmsterdamJanFebMarAprMayJunJulAugSepOctNovDec
AmsterdamJFMAMJJASOND
Sea (°C)87891113161717151310
Sea (°F)464546485255616363595550

When to go

The best time to visit Amsterdam 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, hoping not to have to use them.