We tarp our boat completely for the winter for a couple reasons. Biggest reason is we have lots of big Doug Fir trees in close proximity and it would be covered with needles, cones and small (sometimes large 😳) branches, which makes spring cleaning a MUCH bigger job. Two it keeps the water off the boat which keeps will find its way in to places where it will enevitably freeze at least once each winter, even close to the water in Anacortes. But up in Enumclaw at my parents where we keep it it has snowed (pretty sure) every winter since we have owned it.
100% use a dehumidifier drained out the sink (I know you know why 😉) you need to have power for the dehumidifier, might as well put a heater in it too a good quality heater with overheat protection if you can find one, though not exactly necessary set it pretty cool, you don't need it hot inside just mild 50-65 deg is plenty, to help keep it dry set it in the middle of the floor somewhere. If you choose not to tarp it 100% use a heater I HOP. if you choose not to use a heater at least use a fan to circulate air from the ends of the boat towards the deumidiyer. If you opt to not tarp the boat, I would get some cheap line to use so that you can at least pull all the halyards up to where they enter the mast so there is as little rope exposed to the elements as possible.with your efi motor I would try to flush it with fresh water. A 5 gallon bucket full is enough to run it just make sure to leave it in neutral. I would also make sure the last gas it runs on is non-ethanol and has a fuel stabilizer in it. Check the manual for any special winterization steps. For our carbureted honda we then drain the carb bowl of fuel, I have not been worried about any remaining cooling water left in the system freezing, I think it mostly all drains out anyways.
Batteries... Do not let lithium batteries discharge below their minimum specified voltage, usually around 11.2v, cell damage will result. (it's not going to burn up or anything but it may not be recoverable) . Either charge to full and disconnect it from ALL loads (main battery selector off). Your solar charger probably won't work enough to keep the battery charged if the panel is under a tarp. Better to plug in a lithium compatable smart charger. Generally, the best way to store a lithium battery, long term (generally 6months or longer) is to have your charger hold it at ~80% charge. Check with your battery manufacturer for spacifics. Victron Energy has lots of good info for their batteries and much of it is applicable to LiFPo batteries in general.
Water systems... We do use the RV style potable water antifreeze (generally propylene glycol) not only does it keep the lines from freezing but it also keeps things from growing in the lines as well. Otherwise I would treat the water with sodium hyperchlorate (keeps things from growing and empty the water system as much as possible. In the Anacortes area you are less likely to get a hard freeze over the winter and your heater (you chose to use one right?) should keep everything inside from freezing
Leftover fuel... Use it in your car/truck if compatable, lawnmower or others all engines if not. Start with fresh fuel each season.
It sounds like a bunch of work but on the other hand I would rather do that than clean carbs and fuel lines, replace water lines and pumps, and/or replace ropes and other things sooner than otherwise.
Shaun