Vancouver, where it's located

In Vancouver, the main city of British Columbia, the climate is oceanic, cool and humid, with relatively mild and rainy winters, and cool and fairly sunny summers. The west coast is the only part of Canada in which the average temperature remains above freezing (0 °C or 32 °F) even in winter, and Vancouver is the only major city not having a freezing winter.
Here are the average temperatures.
Vancouver - Average temperatures
Min (°C)1236912141411731
Max (°C)78101317202222191496
Min (°F)343637434854575752453734
Max (°F)454650556368727266574843

Precipitation is quite abundant, given that it amounts to 1,200 millimetres (47 inches) per year, but in the northern suburbs, closer to the mountains, it reaches 1,600 mm (63 in); it is more frequent in the period from November to March; summer is the least rainy season. Vancouver is not exactly on the ocean, since it is protected by the island of the same name, and overlooks the Strait of Georgia. West of the island, on the coast of the Pacific Ocean, it rains much more, since precipitation exceeds 2,500 mm (98 in) per year.
Here is the average precipitation in Vancouver.
Vancouver - Average precipitation

Winter, from December to February, is quite cold, given that the maximum temperatures range between 6 °C and 8 °C (43 °F and 46 °F), but compared to the rest of Canada it's much milder. The temperatures are similar to those of cities such as London and Paris.
The rains are frequent, and during the periods when mild and moist Pacific currents prevail, the temperature can exceed 10 °C (50 °F). On the other hand, in periods of calm weather, fog can form.
On average, it snows for 11 days per year, and 38 centimetres (15 in) of snow per year fall; however, snow usually does not last long, because after a while the mild currents from the ocean start to blow again. Sometimes, colder periods may occur, with frosts of a certain intensity. In December 2008, the temperature dropped to -13 °C (9 °F), and in February 2014 to -9 °C (16 °F).
North of Vancouver, the Coast Mountains, which culminate in Mount Waddington, 4,019 metres (13,186 feet) high, are very snowy and therefore host glaciers and permanent snow already at altitudes of less than 2,000 metres (6,500 feet); in fact the landscape of the area, consisting of fjords and islands, has been shaped over millennia by glaciers.
In winter you can ski in the mountains just outside the city, like Grouse Mountain and Mount Seymour, or in the more northern Coast Mountains; in 2010 Vancouver hosted the Winter Olympic Games.

Spring, from March to May, is initially very cool, if not cold, and rainy, but it gradually becomes milder, although a bit of cold at night is still possible in May. Also, sunny days become gradually more frequent.

Summer, from June to August, is cool, but also quite sunny: the maximum temperatures hover between 20 °C and 23 °C (68 °F and 73 °F), but the Pacific high-pressure system provides longer periods of good weather than in the rest of Canada. If in winter Vancouver is the mildest large city of Canada, in summer it is the coolest, but also the sunniest. On the west coast of Vancouver Island, summer is even cooler: in Tofino the average maximum temperature in July and August is 18/19 °C (64/66 °F).
Unlike in the other seasons, in summer there are several consecutive days of good weather, but there can also be rainy periods, with cloudy skies, rain and cool temperatures, in which highs remain around 15/18 °C (59/64 °F).
Hot days, with highs above 30 °C (86 °F), are rare, and sometimes do not occur for an entire summer.

Autumn, from September to November, is initially mild and pleasant, while starting from October it gradually becomes colder and definitely rainy. November is a very rainy month.

Vancouver from above

The amount of sunshine in Vancouver is scarce in late autumn and winter, when the sky is usually cloudy, while in summer it becomes good, since there are several sunny days.
Vancouver - Sunshine

For swimming, the sea in Vancouver and along the coast of British Columbia is cold even in summer, as you can see in the following table.
Vancouver - Sea temperature
Temp (°C)8881012131414131198
Temp (°F)464646505455575755524846

When to go

The best period to visit Vancouver goes from mid-May to late September, with a preference for July and August, because they are the warmest and driest months.