I’ve recently started doing this on my boat, at least partially. It’s work in progress.
What I did is not really about automating anything. It’s mostly about remote monitoring and surveillance. I had planned to install an RPi as an NMEA multiplexer for a while now, so I just slapped HA alongside with it on the Pi (in a venv). Right now HA monitors the batteries (both engine and house bank), shore power voltage and the solar panels. I also monitor the bilge pump (which is activated automatically over the usual floater switch), as well as ambient temperature and humidity levels.
The Pi connects to the outside world using a 4G modem. I got myself a cheap data plan with 60GB monthly, so that’s plenty. That data is also used for other tasks, getting GRIBs, updating charts and general internet browsing. I also have an Iridium Go when outside of 4G coverage, but I won’t let that anywhere near the Pi
My boat is year round in the water at a marina and it can get pretty damp inside during winter time. I usually have a little portable heater and a dehumidifier for that, which I’d like to both monitor and possibly control through HA as well. But I didn’t get around to do that yet.
Sadly we had some incidents here during the last year, where things got stolen on some boats. This is a very small marina (it’s actually more a small dual use fishing harbour), so there’s very little security around. I’m planning to add a PIR inside the boat and possibly a small camera either inside or outside, which would both connect to HA.
I also added two small tablets for use in the cabins which could access HA, which was fun, but in the end turned out to be pretty useless. HA is pretty much only used for keeping an eye on the boat while I’m not around and sending alerts on my phone if something is not normal.
I thought about connecting HA to the NMEA subsystem at some point. Not for controlling anything, but for gathering statistics and playing around with those. Not sure if I’ll ever get around doing that though, but it sounds like fun.