I seems to me from what I've gathered over the last 30 years that there are numerous
liveaboard opportunities around.
Google & contact Shilshole Bay Marina or Elliot Bay Marina both about 25 minute bike ride mostly on a good bike trail. I think one of them used to allow liveaboards. 48 North is the local free sailors rag and is very helpful,
google them and write a letter to the editor with your query, if published you will reach alot of people, if not contact the editor and you'll likely be helped. Also, around Lake Union and Portage Bay, a 5-25 minute bike ride, there are many mom & pop house boats that could have
liveaboard moorages I would walk/bike/poke around those bodies of
water on a weekend and talk to who ever looks promising. You will get some leads and spend a nice day or two "gunkholing" some of the interesting back waters there. There also is a Nothwest
Marine Trade Industy organization that probably has a listing of
member marinas. There are many smaller marinas other than the 2 I listed above and I am sure some allow liveaboards. Good luck