Diving in the Seattle area is great! Puget Sound has some lovely qualities, among them that it's dead calm more than half of the time. No waves. Another is that there are quite a few easily accessed shore diving sites, many with parking, bathrooms and showers. There is a ton of life in our cold water, although the best sites tend to be where current can be an issue, so timing dives is something you have to know here.
Because of the very large tidal exchanges, and the long narrow nature of the Sound, we have ripping currents in some sites. Reading tide tables and current information, and knowing what the corrections are for the site you contemplate, are necessary things for Sound diving.
We have excellent charters, ranging from relatively customizable 6 packs to large, regular charter boats in the South Sound and the San Juans. Charter fees tend to run about $80 for a two tank day, and unlike SoCal, our boats don't fill and you have to bring two tanks with you.
The water tends to be colder than Southern California, with winter temperatures in the mid 40's and summer temperatures up to the mid 50's. Visibility is better in the winter, but there is more life in the water in the summertime.
If you're contemplating a trip, let us know -- we have a pretty welcoming bunch of divers up here, and somebody can almost always be found to dive with a visitor.