I have owned a boat in this area for many years. I do not use the boat in the winter, as we felt we got enough boating for the 3+ months we were on board every spring/summer, so I "winterized" the boat for the season. I would go "overkill" for what some would think, but I always felt it worked very well for me.
I always drained the freshwater system including the water heater and water tank. I did this more for having a "clean" system more than a freezing prevention measure. Every spring, I would "clean and sanitize" the water system before the season (following Peggy Hall's advice), and we drank the water we had in the system. It always tasted and smelled good.
I posted what I have found to work well (in both a sailboat and my Nordic Tug) over the years in Post #13 (copied below to save anyone having to look back). Neither of these boats had a mildew issue or any "off putting" smells. To achieve that, you must have a warm (doesn't need much),
dry boat.
"I do support the use of a dehumidifier, but you will have to ensure that the on board humidity does not get too low (wooden boat). I would aim for 50-60% humidity. You can use a "home use' "weather station" to obtain readings, and some models of dehumidifiers can be set for a user determined humidity.
added: I used a "home style" dehumidifier bought at Home Depot, Lowes, etc. something like this one - https://www.homedepot.ca/product/to...ith-continuous-operation-function-/1001233850
I also support the engine oil pan heater (about 250 watts so no big draw). It is left on all the time at dock. It keeps the engine warm and dry, helps with circulating warm air in the ER and basically eliminates rust and moisture in the ER. Also, the engine will fire up quickly with "pre-warmed oil" for less wear and tear on start up (it flows easier when warm).
You can use some heat appliances and small fans to move air around to supplement the dehumidifier and reduce any chance of mold/mildew. Heat sources could include oil filled radiators, the small round (some are square) about 100 watt heaters
added link https://www.fisheriessupply.com/caframo-stor-dry-electric-warm-air-circulator-with-fan/9406caabx, etc. to "keep the chill off" if desired. Leave cupboards, interior doors and hatches open, and close up windows and exterior hatches.
Keep track of your amperage draw, and ensure you are not above 20 amps (for 30 amp service) for any sustained period (not just for your pocketbook), but using the above methods, you should not be anywhere near 20 amps continuous draw.
I like a diesel furnace for keeping the boat warm when on board. Espar, Webasto, etc. There are a few options."
The dehumidifier(s) (depending on boat layout, you could use more than one) can be set up to drain overboard using a short hose led to a sink drain, or just need to empty the catchment buckets regularly. When the bucket gets full, the unit shuts off so no "overfilling" and spillage.
Good luck.