In the waters around the BC coast there are a couple of factors that can cause bad Vis for divers. The most common ones are weather/large tidal exchange, silt miners and the dreaded summer phytoplankton bloom.
Tides and windy weather
The bottom composition in most area of coastal BC is such that it is easily disturbed and can float like a cloud in the water column. This is why we see the visibility drop when we have large tidal exchanges or weather with high winds stir up the bottom and create bad visibility.
Another cause is silt miners. These are dive buddies that kick up the bottom causing silt clouds to form. If everybody in your dive group complains about bad vis and you had great vis, then you are a silt miner. Practicing good buoyancy skills can help prevent kicking up silt and disturbing the underwater environment
The third kind of visibility destroyer is the dreaded summer plankton bloom. This is caused by high level of nutrients and lots of sunlight. These conditions can cause the rapid growth of phytoplankton creating clouds that block out visibility and light sometimes up to 60 ft/20m of depth. This creates night dive conditions for scuba divers; it also means to get better visibility you need to decent to deeper waters below the cloud. In 2006 the conditions were so bad that it was visible from space!
A word of caution
There are hundreds of species of phytoplankton, and some of them carry paralytic shellfish-poisoning toxin, known as red tide. If you see signs posted about this heed the warnings and consider skipping the dive in these areas.