It would be good if you could be more specific as to what parameters are most important. You sound like you want to liveaboard
so that will restrict the list of possibilities right away. In Seattle
itself probably the two biggest marinas
are Elliot Bay and Shilshole. You have to drive by the downtown area to get to SeaTac but not bad really as you don't have to get on the interstate. I am not familiar enough with Tacoma to say.
We live in Anacortes which is a couple, or more if traffic is bad which is often the case, of hours minimum to get to SeaTac (assuming this is important). Getting a liveaboard slip
is a haphazard deal though as most places have waiting lists. There are maximums allowed everywhere by state law and marina practices. Wait lists can be short or long and sometimes you just get lucky.
Local knowledge on cruising - that would take an encyclopedia. If you get here you will find enough places to go for ten years without getting bored. But it would be mostly north of the Straits of Juan de Fuca where the San Juans are and then up in to Canadian BC waters. That isn't to say that there aren't many great places to go visit in the lower Sound but it is different and not as spectacular as farther north. And, it gets better the farther north you go. But these trips are in "weeks" not "days", at least from South Sound.
can be important for some. It rains a LOT in Olympia and the weather
is just crappier there than most any other spot around. Port Townsend/Sequim is the driest and sunniest but the most remote
, SeaTac, etc. Anacortes is also good but some distance from Seattle. We are in the rain shadow of the Olympic Mountains so it is much nicer here - but still plenty of green from rain.
So you'll need to really think about what is most important for you. And some marinas have waiting lists just for any kind of slip which varies by size of slip, let alone for liveaboards (if they allow them at all). There are also little hidden marinas that are local knowledge some of which are not as deluxe as the bigger marinas.
Before we moved here we flew in for two weeks, rented a car, and drove north from Seattle hitting as many marinas as we could - Seattle, Everett, Anacortes, Bellingham, Point Roberts, then back south and across the Sound on the ferry
to Port Townsend, Sequim, Port Angeles, and then further south to Gig Harbor, Poulsbo, Bremerton, and Tacoma. We stopped going further south before we got to Bremerton. Our finalists were Bellinham, Anacortes, and Port Townsend. They are all very different and not really "urban" as in big city life (and traffic).
Good luck. You have some homework to do on your own. I would wait on the cruising details until you get here. I promise (if you like our weather and seasons) you will be happy with places to go.