Hi Bus,
Generally, the best diving off our wet coast is in the winter. The visibility is usually good (site or area dependant, of course), it is off season so prices are often a bit lower for hotels and such and shore diving sites tend to be less crowded. However, you really have to like rain - a lot, to appreciate the benefits of west coast winter diving. In the winter we often get one low pressure event after another with little or no break inbetween - it can be a bit soul-sucking for those used to clear skies in winter.
IMO, there are a few "compromise months" - the "shoulder" months of April and May tend to have good weather with still the good vis and off-season benefits typical of winter, as well as September and October for the same reasons. The summer here is usually beautiful, but there are algal blooms, tourist prices and crowds to contend with.
Having said all that, outside of a fall/winter wind storm there is always somewhere to dive off our coast!
Hope that helps.
Lee