For some of us, frost is also an issue. I always have fresh water in the water tanks, I have water taps, septic tank, open seacocks and there is even sea water in the secondary cooling circuit. I keep the frost out through land power and/or the diesel heater, if something should break white I'm out, the result can be a disaster.
Next, battery level needs monitoring all year around. If it falls below 12.3V I should check up and charge. If it falls below 12V it's quite bad. I once had guests, they left a DC/AC inverter on, the water tank was empty and the water pump was going and going and going, and then they found it prudent to disconnect land power when leaving. When I got back, the voltage was 10.5V, and the capacity of the battery bank is really bad after that incident.
Burglar alarm and fire alarm should obviously be included.
Water detection, water can get into the wrong places both due to rain water leakages and sea water leakages. Particularly the latter is of outmost importance to learn early.
GPS position. Nice in case of theft, or for anchor alarms.
I have a raspi that monitors temperature in the cabin and engine room plus humidity and it also takes pictures with a small camera, but I never got time to tinker more with it. It's of no use as long as I don't get alarms.
I have some closed source Raymarine navigation systems that also includes quite some sensors, including radar and depth meadurement. It sucks big time, there is quite much cool one could do with the data.
Thanks for the input. Ya, monitoring for freezing temperatures is another good application. Do you have solar panels and a charge controller for your batteries? Some charge controllers can disconnect loads from the battery when the voltage drops too low to prevent battery damage. GPS position could be monitored via the cell phone portion. That's too bad you can't use the Raymarine data.
I have an overdimensioned Morningstar regulator, haven't checked out if it has such functionality. Still, not all circuits are suitable. The diesel heater should never be disconnected while running (and it will turn itself off if the voltage is too low). Some of the things - for instance the bilge pumps - are supposed to "run to destruction", if the options are between everything sinking and the battery bank getting drained beyond rescue, then ... you get the gist. Anchor winch and bow propeller consumes too much power to run through any delicate electronics, and is also regarded as emergency equipment that shouldn't still work as good as possible, even if the voltage drops a lot - which it will, when pulling lots of amps from the batteries. It's important that the navigation system keeps working even if the voltage should drop.