If you are leaving your boat in the water, in Seattle, it is unlikely that you will see temperatures of below freezing, especially if you have a charger/inverter left on, a water heater left on or some other small source of heat in your ER. If none of those conditions exist, you should put a small electric heater down near the engine(s).
My experience with abandoning a diesel for the winter dates from when I retired, after which, we have become snowbirds. My boat is left totally on her own for almost 6 months every year and has shown no ill effects after 4 repeats, with another few planned, US politics willing.