I don't think you have much to worry about. The water temps will stay above freezing which will keep the boat relatively warm. The only time you have to worry is when a system sets up that dumps cold air in from eastern Washington. When that happens it can get cold (10 F or so) for a few days, but it never lasts longer than a few days. What I would do is not worry except when really cold is forecast. Then I would put a heater on the boat and turn it on. If you aren't going to use the boat for a while, disconnect your fresh water pump and run it dry to get water out of it. Leave all the fresh water taps open. If you are really concerned, close your raw water intake seacock, disconnect the intake hose and run 3 gallons or so of pink antifreeze through the raw water side of the engine. Reconnect the hose and open the seacock. You will be fine.

The vast majority of boats stay in the water all year in the PNW without any heroic issues. Remember 35 is cold on Puget Sound in the winter. Most of the time when it goes below freezing it is only for a couple of hours in the very early morning.

Note - I grew up boating on Puget Sound.