Hi
We use our boat during the winter too, even if we don’t go out due to weather I regulary start engine & run boat in gear against mooring lines for about 10 mins.
(diesel engines dont like running without any load)
Probably the most important thing I do is turn off the pressurised fresh water pump & release all pressure from system, (open tap & let drain out).
This prevents your bathing platform / stern shower from freezing up & then when it thaws filling your bilges with water.
Lastly keep your fuel tank as full as possible to avoid excess condensation forming inside.
